Fair Value Measurements - Schedule of Available-for-sale Investments (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Short-term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Amortized Cost	$ 36,195	$ 42,571
Gross Unrealized Gains	14
Gross Unrealized Losses	(11)	(61)
Market Value	36,198	42,510
Short-term Investments [Member] | United States Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Amortized Cost	5,004
Gross Unrealized Gains	4
Market Value	5,008
Short-term Investments [Member] | Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Amortized Cost	19,216	39,575
Gross Unrealized Gains	10
Gross Unrealized Losses	(11)	(61)
Market Value	19,215	39,514
Short-term Investments [Member] | Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Amortized Cost	11,975	2,996
Market Value	11,975	2,996
Long-term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Amortized Cost	10,325	29,924
Gross Unrealized Gains	5
Gross Unrealized Losses	(1)	(110)
Market Value	10,329	29,814
Long-term Investments [Member] | United States Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Amortized Cost		2,006
Gross Unrealized Losses		(7)
Market Value		1,999
Long-term Investments [Member] | Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Amortized Cost	3,320	16,990
Gross Unrealized Losses	(1)	(87)
Market Value	3,319	16,903
Long-term Investments [Member] | Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Amortized Cost	7,005	10,928
Gross Unrealized Gains	5
Gross Unrealized Losses		(16)
Market Value	$ 7,010	$ 10,912