Investments - Schedule of Fair Value of Available-for-Sale Investments by Type of Security (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	$ 160,423	$ 175,603
Gross Unrealized Gain	31	59
Gross Unrealized Loss	(181)	(206)
Fair Value	160,273	175,456
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	24,151	19,631
Gross Unrealized Gain	31	58
Fair Value	24,182	19,689
Certificates of Deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	10,382	10,629
Fair Value	10,382	10,629
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	76,787	94,764
Gross Unrealized Loss	(103)	(155)
Fair Value	76,684	94,609
Government Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	33,005	33,513
Gross Unrealized Loss	(65)	(47)
Fair Value	32,940	33,466
Treasury Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	16,098	17,066
Gross Unrealized Gain		1
Gross Unrealized Loss	(13)	(4)
Fair Value	$ 16,085	$ 17,063