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
Jun. 30, 2017
Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost $ 123,702 $ 105,365
Fair Value 123,658 104,929
Short-term investments [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 72,488 27,484
Gross Unrealized Gains 0 0
Gross Unrealized Losses (155) (30)
Fair Value 72,333 27,454
Short-term investments [Member] | U.S. Treasuries [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 59,044 24,987
Gross Unrealized Gains 0 0
Gross Unrealized Losses (141) (30)
Fair Value 58,903 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 (12) 0
Fair Value 7,004 2,497
Short-term investments [Member] | Corporate debt securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 6,428 0
Gross Unrealized Gains 0 0
Gross Unrealized Losses (2) 0
Fair Value 6,426 0
Long-term marketable securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 51,214 77,881
Gross Unrealized Gains 338 92
Gross Unrealized Losses (227) (498)
Fair Value 51,325 77,475
Long-term marketable securities [Member] | U.S. Treasuries [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 16,338 40,865
Gross Unrealized Gains 11 0
Gross Unrealized Losses (114) (229)
Fair Value 16,235 40,636
Long-term marketable securities [Member] | U.S. Government Sponsored Entities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 5,454 12,618
Gross Unrealized Gains 0 0
Gross Unrealized Losses (52) (58)
Fair Value 5,402 12,560
Long-term marketable securities [Member] | Corporate debt securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 22,531 17,841
Gross Unrealized Gains 291 74
Gross Unrealized Losses (48) (165)
Fair Value 22,774 17,750
Long-term marketable securities [Member] | Asset-backed securities and other [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 6,891 6,557
Gross Unrealized Gains 36 18
Gross Unrealized Losses (13) (46)
Fair Value $ 6,914 $ 6,529