Short-term Investments - Short-Term Investments Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	$ 36,813	$ 55,045
Gross unrealized gains	0	2
Gross unrealized losses	(29)	(45)
Fair value	36,784	55,002
U.S. government-related debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost		4,502
Gross unrealized gains		0
Gross unrealized losses		(4)
Fair value		4,498
Corporate debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	35,419	47,345
Gross unrealized gains	0	2
Gross unrealized losses	(29)	(40)
Fair value	35,390	47,307
Asset-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	1,394	3,198
Gross unrealized gains	0	0
Gross unrealized losses	0	(1)
Fair value	$ 1,394	$ 3,197