Quarterly report pursuant to Section 13 or 15(d)

Investments in Marketable Securities - Schedule of Amortized Cost and Fair Value of Marketable Securities, Available-for-Sale, by Type of Security (Detail)

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Investments in Marketable Securities - Schedule of Amortized Cost and Fair Value of Marketable Securities, Available-for-Sale, by Type of Security (Detail) - USD ($)
$ in Thousands
Mar. 31, 2017
Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost $ 123,713 $ 105,365
Fair Value 123,383 104,929
Short-term investments [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 57,005 27,484
Gross Unrealized Gains 4 0
Gross Unrealized Losses (99) (30)
Fair Value 56,910 27,454
Short-term investments [Member] | U.S. Treasuries [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 48,604 24,987
Gross Unrealized Gains 2 0
Gross Unrealized Losses (93) (30)
Fair Value 48,513 24,957
Short-term investments [Member] | U.S. Government Sponsored Entities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 1,998 2,497
Gross Unrealized Gains 0 0
Gross Unrealized Losses (2) 0
Fair Value 1,996 2,497
Short-term investments [Member] | Corporate debt securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 6,403 0
Gross Unrealized Gains 2 0
Gross Unrealized Losses (4) 0
Fair Value 6,401 0
Long-term marketable securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 66,708 77,881
Gross Unrealized Gains 130 92
Gross Unrealized Losses (365) (498)
Fair Value 66,473 77,475
Long-term marketable securities [Member] | U.S. Treasuries [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 24,539 40,865
Gross Unrealized Gains 0 0
Gross Unrealized Losses (162) (229)
Fair Value 24,377 40,636
Long-term marketable securities [Member] | U.S. Government Sponsored Entities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 12,553 12,618
Gross Unrealized Gains 0 0
Gross Unrealized Losses (67) (58)
Fair Value 12,486 12,560
Long-term marketable securities [Member] | Corporate debt securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 22,526 17,841
Gross Unrealized Gains 108 74
Gross Unrealized Losses (104) (165)
Fair Value 22,530 17,750
Long-term marketable securities [Member] | Asset-backed securities and other [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 7,090 6,557
Gross Unrealized Gains 22 18
Gross Unrealized Losses (32) (46)
Fair Value $ 7,080 $ 6,529