Annual report pursuant to Section 13 and 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
Dec. 31, 2017
Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost $ 125,761 $ 105,365
Fair Value 125,659 104,929
Short-term investments [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 73,659 27,484
Gross Unrealized Gains 0 0
Gross Unrealized Losses (99) (30)
Fair Value 73,560 27,454
Short-term investments [Member] | U.S. Treasuries [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 57,712 24,987
Gross Unrealized Gains 0 0
Gross Unrealized Losses (88) (30)
Fair Value 57,624 24,957
Short-term investments [Member] | U.S. Government Sponsored Entities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 7,016 2,497
Gross Unrealized Gains 0 0
Gross Unrealized Losses (8) 0
Fair Value 7,008 2,497
Short-term investments [Member] | Corporate debt securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 8,931 0
Gross Unrealized Gains 0 0
Gross Unrealized Losses (3) 0
Fair Value 8,928 0
Long-term marketable securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 52,102 77,881
Gross Unrealized Gains 292 92
Gross Unrealized Losses (295) (498)
Fair Value 52,099 77,475
Long-term marketable securities [Member] | U.S. Treasuries [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 18,111 40,865
Gross Unrealized Gains 7 0
Gross Unrealized Losses (164) (229)
Fair Value 17,954 40,636
Long-term marketable securities [Member] | U.S. Government Sponsored Entities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 5,306 12,618
Gross Unrealized Gains 0 0
Gross Unrealized Losses (62) (58)
Fair Value 5,244 12,560
Long-term marketable securities [Member] | Corporate debt securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 22,505 17,841
Gross Unrealized Gains 268 74
Gross Unrealized Losses (54) (165)
Fair Value 22,719 17,750
Long-term marketable securities [Member] | Asset-backed securities and other [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 6,180 6,557
Gross Unrealized Gains 17 18
Gross Unrealized Losses (15) (46)
Fair Value $ 6,182 $ 6,529