Fair Value Measurements - Summary of Estimated Value of Cash Equivalents and Marketable Securities and Gross Unrealized Holding Gains and Losses (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents, Amortized cost	$ 74,932	$ 150,456
Cash and cash equivalents, Estimated Fair Value	74,932	150,456
Marketable securities, Amortized cost	287,115	280,770
Unrealized gains	7	41
Unrealized losses	(148)	(222)
Marketable securities, Estimated Fair Value	286,974	280,589
Cash
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents, Amortized cost	13,265	15,175
Cash and cash equivalents, Estimated Fair Value	13,265	15,175
Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents, Amortized cost	54,318	104,602
Cash and cash equivalents, Estimated Fair Value	54,318	104,602
Commercial Paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents, Amortized cost	7,349	7,899
Cash and cash equivalents, Estimated Fair Value	7,349	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		22,780
Cash and cash equivalents, Estimated Fair Value		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	166,854	171,416
Unrealized gains	7	3
Unrealized losses	(61)	(144)
Marketable securities, Estimated Fair Value	166,800	171,275
Corporate Debt Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities, Amortized cost	77,967	74,958
Unrealized gains		38
Unrealized losses	(87)	(78)
Marketable securities, Estimated Fair Value	77,880	74,918
Commercial paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities, Amortized cost	42,294	34,396
Marketable securities, Estimated Fair Value	$ 42,294	$ 34,396