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
Sep. 30, 2017
Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost $ 128,131 $ 105,365
Fair Value 128,257 104,929
Short-term investments [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 82,913 27,484
Gross Unrealized Gains 0 0
Gross Unrealized Losses (100) (30)
Fair Value 82,813 27,454
Short-term investments [Member] | U.S. Treasuries [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 65,457 24,987
Gross Unrealized Gains 0 0
Gross Unrealized Losses (92) (30)
Fair Value 65,365 24,957
Short-term investments [Member] | U.S. Government Sponsored Entities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 7,034 2,497
Gross Unrealized Gains 0 0
Gross Unrealized Losses (8) 0
Fair Value 7,026 2,497
Short-term investments [Member] | Corporate debt securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 10,422 0
Gross Unrealized Gains 0 0
Gross Unrealized Losses 0 0
Fair Value 10,422 0
Long-term marketable securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 45,218 77,881
Gross Unrealized Gains 409 92
Gross Unrealized Losses (183) (498)
Fair Value 45,444 77,475
Long-term marketable securities [Member] | U.S. Treasuries [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 10,062 40,865
Gross Unrealized Gains 12 0
Gross Unrealized Losses (99) (229)
Fair Value 9,975 40,636
Long-term marketable securities [Member] | U.S. Government Sponsored Entities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 5,372 12,618
Gross Unrealized Gains 0 0
Gross Unrealized Losses (45) (58)
Fair Value 5,327 12,560
Long-term marketable securities [Member] | Corporate debt securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 22,503 17,841
Gross Unrealized Gains 358 74
Gross Unrealized Losses (29) (165)
Fair Value 22,832 17,750
Long-term marketable securities [Member] | Asset-backed securities and other [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 7,281 6,557
Gross Unrealized Gains 39 18
Gross Unrealized Losses (10) (46)
Fair Value $ 7,310 $ 6,529