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, 2018
Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost $ 206,975 $ 125,761
Fair Value 205,836 125,659
Short-term investments [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 120,881 73,659
Gross Unrealized Gains 0 0
Gross Unrealized Losses (180) (99)
Fair Value 120,701 73,560
Short-term investments [Member] | U.S. Treasuries [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 106,291 57,712
Gross Unrealized Gains 0 0
Gross Unrealized Losses (150) (88)
Fair Value 106,141 57,624
Short-term investments [Member] | U.S. Government Sponsored Entities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 3,502 7,016
Gross Unrealized Gains 0 0
Gross Unrealized Losses (17) (8)
Fair Value 3,485 7,008
Short-term investments [Member] | Corporate debt securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 10,988 8,931
Gross Unrealized Gains 0 0
Gross Unrealized Losses (13) (3)
Fair Value 10,975 8,928
Short-term investments [Member] | Asset-backed securities and other [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 100 0
Gross Unrealized Gains 0 0
Gross Unrealized Losses 0 0
Fair Value 100 0
Long-term marketable securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 86,094 52,102
Gross Unrealized Gains 5 292
Gross Unrealized Losses (964) (295)
Fair Value 85,135 52,099
Long-term marketable securities [Member] | U.S. Treasuries [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 52,865 18,111
Gross Unrealized Gains 0 7
Gross Unrealized Losses (343) (164)
Fair Value 52,522 17,954
Long-term marketable securities [Member] | U.S. Government Sponsored Entities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 1,603 5,306
Gross Unrealized Gains 0 0
Gross Unrealized Losses (83) (62)
Fair Value 1,520 5,244
Long-term marketable securities [Member] | Corporate debt securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 25,374 22,505
Gross Unrealized Gains 4 268
Gross Unrealized Losses (471) (54)
Fair Value 24,907 22,719
Long-term marketable securities [Member] | Asset-backed securities and other [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 6,252 6,180
Gross Unrealized Gains 1 17
Gross Unrealized Losses (67) (15)
Fair Value $ 6,186 $ 6,182