Marketable Securities - Summary of Marketable Securities Designated as Available-for-Sale (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
Corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	131,740	99,592
Gross Unrealized Gains	6	1
Gross Unrealized Losses	(116)	(119)
Estimated Fair Value	131,630	99,474
Municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	17,957	18,146
Gross Unrealized Gains	14	1
Gross Unrealized Losses	(1)	(10)
Estimated Fair Value	17,970	18,137
Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	36,118	9,196
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	36,118	9,196
Agency securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	7,279
Gross Unrealized Gains	7
Gross Unrealized Losses	0
Estimated Fair Value	7,286
Certificates of deposit
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,492	498
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	2,492	$ 498
Government securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,199
Gross Unrealized Gains	2
Gross Unrealized Losses	0
Estimated Fair Value	$ 1,201