Investments - Schedule of Fair Value of Available-for-Sale Investments by Type of Security (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	$ 109,941	$ 131,116
Gross Unrealized Gain	109	87
Gross Unrealized Loss		(54)
Fair Value	110,050	131,149
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	55,928	64,733
Gross Unrealized Gain	92	87
Fair Value	56,020	64,820
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	26,964	46,538
Gross Unrealized Gain	8
Gross Unrealized Loss		(48)
Fair Value	26,972	46,490
Government Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	15,037	15,823
Gross Unrealized Gain	7
Gross Unrealized Loss		(4)
Fair Value	15,044	15,819
Treasury Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	12,012	4,022
Gross Unrealized Gain	2
Gross Unrealized Loss		(2)
Fair Value	$ 12,014	$ 4,020