Investments in Marketable Securities (Tables)
   | 
12 Months Ended | 
Dec. 31, 2018  | 
| Investments, Debt and Equity Securities [Abstract] | 
 
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| Schedule of Amortized Cost and Fair Value of Marketable Securities, Available-for-Sale, by Type of Security | 
 
 Amortized cost and fair value of marketable securities,
 available-for-sale, by type of
 security consisted of the following (in thousands): 
 
   
 
 
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 December 31, 2018 | 
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 December 31, 2017 | 
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 |   | 
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 Amortized 
 Cost | 
   | 
    | 
 Gross 
 Unrealized 
 Gains | 
   | 
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 Gross 
 Unrealized 
 Losses | 
   | 
   | 
 
Fair 
 Value
 | 
   | 
    | 
 Amortized 
 Cost | 
   | 
    | 
 Gross 
 Unrealized 
 Gains | 
   | 
    | 
 Gross 
 Unrealized 
 Losses | 
   | 
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Fair 
 Value
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 Short-term investments: 
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 U.S. treasuries 
  | 
    | 
 $ | 
 121,252 | 
   | 
    | 
 $ | 
 7 | 
   | 
    | 
 $ | 
 (79 | 
 )  | 
   | 
 $ | 
 121,180 | 
   | 
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 $ | 
 57,712 | 
   | 
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 $ | 
 
 —   | 
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 $ | 
 (88 | 
 )  | 
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 $ | 
 57,624 | 
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 U.S. government sponsored entities 
  | 
    | 
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 3,512 | 
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 —   | 
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 (7 | 
 )  | 
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 3,505 | 
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 7,016 | 
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 —   | 
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 (8 | 
 )  | 
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 7,008 | 
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 Corporate debt securities 
  | 
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 11,962 | 
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 —   | 
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 (11 | 
 )  | 
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 11,951 | 
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 8,931 | 
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 —   | 
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 (3 | 
 )  | 
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 8,928 | 
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 Asset-backed securities and other 
  | 
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 806 | 
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 —   | 
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 (6 | 
 )  | 
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 800 | 
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 —   | 
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 —   | 
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 $ | 
 137,532 | 
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 $ | 
 7 | 
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 $ | 
 (103 | 
 )  | 
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 $ | 
 137,436 | 
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 $ | 
 73,659 | 
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 $ | 
 
 —   | 
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 $ | 
 (99 | 
 )  | 
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 $ | 
 73,560 | 
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 Long-term investments: 
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 U.S. treasuries 
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 $ | 
 44,997 | 
   | 
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 $ | 
 128 | 
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 $ | 
 (115 | 
 )  | 
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 $ | 
 45,010 | 
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 $ | 
 18,111 | 
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 $ | 
 7 | 
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 $ | 
 (164 | 
 )  | 
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 $ | 
 17,954 | 
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 U.S. government sponsored entities 
  | 
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 1,569 | 
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 —   | 
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 (62 | 
 )  | 
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 1,507 | 
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 5,306 | 
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 —   | 
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 (62 | 
 )  | 
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 5,244 | 
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 Corporate debt securities 
  | 
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 32,467 | 
   | 
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 3 | 
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 (633 | 
 )  | 
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 31,837 | 
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 22,505 | 
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 268 | 
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 (54 | 
 )  | 
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 22,719 | 
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 Asset-backed securities and other 
  | 
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 4,889 | 
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 12 | 
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 (46 | 
 )  | 
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 4,855 | 
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 6,180 | 
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 17 | 
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 (15 | 
 )  | 
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 6,182 | 
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 $ | 
 83,922 | 
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 $ | 
 143 | 
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 $ | 
 (856 | 
 )  | 
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 $ | 
 83,209 | 
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 $ | 
 52,102 | 
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 $ | 
 292 | 
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 $ | 
 (295 | 
 )  | 
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 $ | 
 52,099 | 
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| Amortized Cost and Fair Value of Investments in Available for Sale Securities | 
 
 The amortized cost and fair value of the Company’s
 investments in available-for-sale securities
 that have been in a continuous unrealized loss position consisted
 of the following (in thousands): 
 
   
 
 
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    | 
 December 31, 2018 | 
   | 
    | 
 December 31, 2017 | 
   | 
  
 
 |   | 
    | 
 Unrealized 
 Loss | 
   | 
    | 
 Fair Value | 
   | 
    | 
 Unrealized 
 Loss | 
   | 
    | 
 Fair 
 Value | 
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 Less than 12 months 
  | 
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 $ | 
 (576 | 
 )  | 
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 $ | 
 127,326 | 
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 $ | 
 (158 | 
 )  | 
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 $ | 
 63,229 | 
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 12 months or longer 
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 $ | 
 (383 | 
 )  | 
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 $ | 
 30,609 | 
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 $ | 
 (236 | 
 )  | 
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 $ | 
 44,961 | 
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| Gross Realized Gains and Losses from Sale of Available for Sale Securities | 
 
 Gross realized gains and gross realized losses from the sales of
 the Company’s available-for-sale securities
 consisted of the following (in thousands): 
 
   
 
 
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 December 31, | 
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 2018 | 
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 2017 | 
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 Gross realized gains(1) 
  | 
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 $ | 
 12 | 
   | 
    | 
 $ | 
 2 | 
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 Gross realized losses(1) 
  | 
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 $ | 
 (2 | 
 )  | 
    | 
 $ | 
 
 —   | 
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 | (1) | 
 
  Recorded in other income (expense), net in the
 consolidated statements of net and comprehensive income. The cost
 basis of securities sold were determined based on the specific
 identification method. 
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| Schedule of Amortized Cost and Fair Value of Marketable Securities, Available-for-Sale, by Contractual Maturity | 
 
 Amortized cost and fair value of marketable securities,
 available-for-sale, by
 contractual maturity consisted of the following (in thousands): 
 
   
 
 
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 |   | 
    | 
 December 31, 2018 | 
   | 
    | 
 December 31, 2017 | 
   | 
  
 
 |   | 
    | 
 Amortized 
 Cost | 
   | 
    | 
 Fair Value | 
   | 
    | 
 Amortized 
 Cost | 
   | 
    | 
 Fair Value | 
   | 
  
 
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 Due in one year or less 
  | 
    | 
 $ | 
 137,532 | 
   | 
    | 
 $ | 
 137,436 | 
   | 
    | 
 $ | 
 73,659 | 
   | 
    | 
 $ | 
 73,560 | 
   | 
  
 
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 Due after one year through five years 
  | 
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 61,875 | 
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 61,846 | 
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 30,644 | 
   | 
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 30,517 | 
   | 
  
 
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 Due after five years through ten years 
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 17,310 | 
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 16,747 | 
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 15,090 | 
   | 
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 15,200 | 
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 Due after ten years 
  | 
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 4,737 | 
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 4,616 | 
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 6,368 | 
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 6,382 | 
   | 
  
 
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    | 
 $ | 
 221,454 | 
   | 
    | 
 $ | 
 220,645 | 
   | 
    | 
 $ | 
 125,761 | 
   | 
    | 
 $ | 
 125,659 | 
   | 
  
 
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 Weighted average contractual maturity 
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 1.8 years | 
   | 
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 2.6 years | 
   | 
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