Fair Value Measurements - Summary of Estimated Value of Cash Equivalents and Marketable Securities and Gross Unrealized Holding Gains and Losses (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents, Amortized cost	$ 120,133	$ 150,456
Cash and cash equivalents, Estimated Fair Value	120,133	150,456
Marketable securities, Amortized cost	288,274	280,770
Unrealized gains	73	41
Unrealized losses	(20)	(222)
Marketable securities, Estimated Fair Value	288,327	280,589
Cash
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents, Amortized cost	13,185	15,175
Cash and cash equivalents, Estimated Fair Value	13,185	15,175
Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents, Amortized cost	85,451	104,602
Cash and cash equivalents, Estimated Fair Value	85,451	104,602
Commercial Paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents, Amortized cost	19,498	7,899
Cash and cash equivalents, Estimated Fair Value	19,498	7,899
U.S. government and agency securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents, Amortized cost	1,999	22,780
Cash and cash equivalents, Estimated Fair Value	1,999	22,780
U S Government Agencies Debt Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities, Amortized cost	189,226	171,416
Unrealized gains	48	3
Unrealized losses	(12)	(144)
Marketable securities, Estimated Fair Value	189,262	171,275
Corporate Debt Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities, Amortized cost	63,656	74,958
Unrealized gains	25	38
Unrealized losses	(8)	(78)
Marketable securities, Estimated Fair Value	63,673	74,918
Commercial paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities, Amortized cost	35,392	34,396
Marketable securities, Estimated Fair Value	$ 35,392	$ 34,396