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, 2018
Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost $ 131,152 $ 125,761
Fair Value 130,399 125,659
Short-term investments [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 95,045 73,659
Gross Unrealized Gains 0 0
Gross Unrealized Losses (219) (99)
Fair Value 94,826 73,560
Short-term investments [Member] | U.S. Treasuries [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 72,344 57,712
Gross Unrealized Gains 0 0
Gross Unrealized Losses (171) (88)
Fair Value 72,173 57,624
Short-term investments [Member] | U.S. Government Sponsored Entities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 10,536 7,016
Gross Unrealized Gains 0 0
Gross Unrealized Losses (35) (8)
Fair Value 10,501 7,008
Short-term investments [Member] | Corporate debt securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 12,165 8,931
Gross Unrealized Gains 0 0
Gross Unrealized Losses (13) (3)
Fair Value 12,152 8,928
Long-term marketable securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 36,107 52,102
Gross Unrealized Gains 37 292
Gross Unrealized Losses (571) (295)
Fair Value 35,573 52,099
Long-term marketable securities [Member] | U.S. Treasuries [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 4,553 18,111
Gross Unrealized Gains 0 7
Gross Unrealized Losses (160) (164)
Fair Value 4,393 17,954
Long-term marketable securities [Member] | U.S. Government Sponsored Entities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 1,737 5,306
Gross Unrealized Gains 0 0
Gross Unrealized Losses (67) (62)
Fair Value 1,670 5,244
Long-term marketable securities [Member] | Corporate debt securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 23,332 22,505
Gross Unrealized Gains 28 268
Gross Unrealized Losses (304) (54)
Fair Value 23,056 22,719
Long-term marketable securities [Member] | Asset-backed securities and other [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 6,485 6,180
Gross Unrealized Gains 9 17
Gross Unrealized Losses (40) (15)
Fair Value $ 6,454 $ 6,182