Fair Value Measurements - Summary of Estimated Value of Cash Equivalents and Marketable Securities and Gross Unrealized Holding Gains and Losses (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014	Dec. 31, 2013
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents, Amortized cost	$ 180,991	$ 119,456	$ 39,551	$ 8,532
Cash and cash equivalents, Estimated Fair Value	180,991
Marketable securities, Amortized cost	267,346
Unrealized gains	100
Unrealized losses	(25)
Marketable securities, Estimated Fair Value	267,421
Cash
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents, Amortized cost	5,972
Cash and cash equivalents, Estimated Fair Value	5,972
Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents, Amortized cost	116,642
Cash and cash equivalents, Estimated Fair Value	116,642
Commercial Paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents, Amortized cost	24,998
Cash and cash equivalents, Estimated Fair Value	24,998
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	23,658
Cash and cash equivalents, Estimated Fair Value	23,658
Corporate Debt Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents, Amortized cost	9,721
Cash and cash equivalents, Estimated Fair Value	9,721
Marketable securities, Amortized cost	73,536
Unrealized gains	36
Unrealized losses	(25)
Marketable securities, Estimated Fair Value	73,547
U S Government Agencies Debt Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities, Amortized cost	180,344
Unrealized gains	64
Marketable securities, Estimated Fair Value	180,408
Commercial paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities, Amortized cost	13,466
Marketable securities, Estimated Fair Value	$ 13,466