Summary of Significant Accounting Policies - Summary of Marketable Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 136,607	$ 57,394
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(60)	(35)
Fair Value	136,547	57,359
U.S. Government Securities (Due within 1 Year) [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	49,240	34,200
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(1)	(9)
Fair Value	49,239	34,191
Corporate Bonds (Due within 1 Year) [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	57,396	18,716
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(48)	(18)
Fair Value	57,348	18,698
Corporate Bonds (Due After 1 Year Through 2 Years) [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	3,487	3,478
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(4)	(8)
Fair Value	3,483	3,470
Commercial Paper (Due within 1 Year) [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	26,484	1,000
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(7)
Fair Value	$ 26,477	$ 1,000

Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

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