Investments - Schedule of Fair Value of Available-for-Sale Investments by Type of Security (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	$ 175,603	$ 131,116
Gross Unrealized Gain	59	87
Gross Unrealized Loss	(206)	(54)
Fair Value	175,456	131,149
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	19,631	64,733
Gross Unrealized Gain	58	87
Fair Value	19,689	64,820
Certificates of Deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	10,629
Fair Value	10,629
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	94,764	46,538
Gross Unrealized Loss	(155)	(48)
Fair Value	94,609	46,490
Government Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	33,513	15,823
Gross Unrealized Loss	(47)	(4)
Fair Value	33,466	15,819
Treasury Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	17,066	4,022
Gross Unrealized Gain	1
Gross Unrealized Loss	(4)	(2)
Fair Value	$ 17,063	$ 4,020