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, 2018
Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost $ 221,454 $ 125,761
Fair Value 220,645 125,659
Short-term investments [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 137,532 73,659
Gross Unrealized Gains 7 0
Gross Unrealized Losses (103) (99)
Fair Value 137,436 73,560
Short-term investments [Member] | U.S. Treasuries [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 121,252 57,712
Gross Unrealized Gains 7 0
Gross Unrealized Losses (79) (88)
Fair Value 121,180 57,624
Short-term investments [Member] | U.S. Government Sponsored Entities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 3,512 7,016
Gross Unrealized Gains 0 0
Gross Unrealized Losses (7) (8)
Fair Value 3,505 7,008
Short-term investments [Member] | Corporate debt securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 11,962 8,931
Gross Unrealized Gains 0 0
Gross Unrealized Losses (11) (3)
Fair Value 11,951 8,928
Short-term investments [Member] | Asset-backed securities and other [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 806 0
Gross Unrealized Gains 0 0
Gross Unrealized Losses (6) 0
Fair Value 800 0
Long-term marketable securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 83,922 52,102
Gross Unrealized Gains 143 292
Gross Unrealized Losses (856) (295)
Fair Value 83,209 52,099
Long-term marketable securities [Member] | U.S. Treasuries [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 44,997 18,111
Gross Unrealized Gains 128 7
Gross Unrealized Losses (115) (164)
Fair Value 45,010 17,954
Long-term marketable securities [Member] | U.S. Government Sponsored Entities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 1,569 5,306
Gross Unrealized Gains 0 0
Gross Unrealized Losses (62) (62)
Fair Value 1,507 5,244
Long-term marketable securities [Member] | Corporate debt securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 32,467 22,505
Gross Unrealized Gains 3 268
Gross Unrealized Losses (633) (54)
Fair Value 31,837 22,719
Long-term marketable securities [Member] | Asset-backed securities and other [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 4,889 6,180
Gross Unrealized Gains 12 17
Gross Unrealized Losses (46) (15)
Fair Value $ 4,855 $ 6,182