Marketable Securities - Summary of Amortized Cost and Estimated Fair Value of Marketable Securities by Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 196,785	$ 127,432
Gross Unrealized Gains	29	2
Gross Unrealized Losses	(117)	(129)
Estimated Fair Value	196,697	127,305
Due in one year or less
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	147,745	127,029
Gross Unrealized Gains	13	2
Gross Unrealized Losses	(75)	(129)
Estimated Fair Value	147,683	126,902
Due after one year
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	49,040	403
Gross Unrealized Gains	16	0
Gross Unrealized Losses	(42)	0
Estimated Fair Value	$ 49,014	$ 403