Investments - Summary of Cash, Cash Equivalents and Investments (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014	Jun. 30, 2014	Dec. 31, 2013
Cash and Cash Equivalents [Abstract]
Amortized Cost	$ 100,916	$ 85,586	$ 27,876	$ 69,649
Fair Value	100,916	85,586
Investments [Abstract]
Amortized Cost	162,970	158,307
Gross Unrealized Gains	9	2
Gross Unrealized Losses	(50)	(109)
Fair Value	162,929	158,200
Cash and money market accounts
Cash and Cash Equivalents [Abstract]
Amortized Cost	100,916	84,613
Fair Value	100,916	84,613
Certificates of deposit
Cash and Cash Equivalents [Abstract]
Amortized Cost		472
Fair Value		472
Corporate bonds
Cash and Cash Equivalents [Abstract]
Amortized Cost		501
Fair Value		501
Certificates of deposit (due within 1 year)
Investments [Abstract]
Amortized Cost	24,190	25,823
Gross Unrealized Gains	5	0
Gross Unrealized Losses	(1)	(9)
Fair Value	24,194	25,814
Certificates of Deposit, Due Within Two Years [Member]
Investments [Abstract]
Amortized Cost	3,120	4,429
Gross Unrealized Gains	2	1
Gross Unrealized Losses	(2)	(3)
Fair Value	3,120	4,427
Commercial paper
Investments [Abstract]
Amortized Cost	5,989	5,988
Gross Unrealized Gains	1	1
Gross Unrealized Losses	0	(3)
Fair Value	5,990	5,986
Corporate bonds (due within 1 year)
Investments [Abstract]
Amortized Cost	23,203	16,487
Gross Unrealized Gains	1	0
Gross Unrealized Losses	(44)	(24)
Fair Value	23,160	16,463
Corporate Bonds, Due within 2 years [Member]
Investments [Abstract]
Amortized Cost	1,022	13,912
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(3)	(64)
Fair Value	1,019	13,848
US Government Corporations and Agencies Securities [Member]
Investments [Abstract]
Amortized Cost	4,530	6,082
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	(6)
Fair Value	4,530	6,076
Total investments
Investments [Abstract]
Amortized Cost	62,054	72,721
Gross Unrealized Gains	9	2
Gross Unrealized Losses	(50)	(109)
Fair Value	$ 62,013	$ 72,614