Fair Value Measurements - Summary of Estimated Value of Cash, Cash Equivalents and Marketable Securities and Gross Unrealized Holding Gains and Losses (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents, Amortized cost	$ 97,983	$ 150,456
Cash and cash equivalents, Estimated Fair Value	97,983	150,456
Marketable securities, Amortized cost	289,140	280,770
Unrealized gains	71	41
Unrealized losses	(47)	(222)
Marketable securities, Estimated Fair Value	289,164	280,589
Cash
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents, Amortized cost	14,987	15,175
Cash and cash equivalents, Estimated Fair Value	14,987	15,175
Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents, Amortized cost	57,500	104,602
Cash and cash equivalents, Estimated Fair Value	57,500	104,602
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	4,700	22,780
Cash and cash equivalents, Estimated Fair Value	4,700	22,780
Commercial Paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents, Amortized cost	20,796	7,899
Cash and cash equivalents, Estimated Fair Value	20,796	7,899
U S Government Agencies Debt Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities, Amortized cost	182,206	171,416
Unrealized gains	65	3
Unrealized losses	(23)	(144)
Marketable securities, Estimated Fair Value	182,248	171,275
Corporate Debt Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities, Amortized cost	68,209	74,958
Unrealized gains	6	38
Unrealized losses	(24)	(78)
Marketable securities, Estimated Fair Value	68,191	74,918
Commercial paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities, Amortized cost	38,725	34,396
Marketable securities, Estimated Fair Value	$ 38,725	$ 34,396