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, 2019	Dec. 31, 2018
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents, Amortized cost	$ 53,606	$ 126,313
Cash and cash equivalents, Unrealized losses	(1)	(3)
Cash and cash equivalents, Estimated Fair Value	53,605	126,310
Marketable securities, Amortized cost	181,626	151,735
Marketable securities, Unrealized gains	157	19
Marketable securities, Unrealized losses	(8)	(191)
Marketable securities, Estimated Fair Value	181,775	151,563
Cash
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents, Amortized cost	11,073	36,124
Cash and cash equivalents, Estimated Fair Value	11,073	36,124
Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents, Amortized cost	28,100	22,082
Cash and cash equivalents, Estimated Fair Value	28,100	22,082
Commercial Paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents, Amortized cost	12,933	62,413
Cash and cash equivalents, Unrealized losses	(1)
Cash and cash equivalents, Estimated Fair Value	12,932	62,413
Corporate Debt Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents, Amortized cost	1,500	5,694
Cash and cash equivalents, Unrealized losses		(3)
Cash and cash equivalents, Estimated Fair Value	1,500	5,691
Marketable securities, Amortized cost	78,763	65,319
Marketable securities, Unrealized gains	81	3
Marketable securities, Unrealized losses	(3)	(49)
Marketable securities, Estimated Fair Value	78,841	65,273
U.S. Government and Agency Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities, Amortized cost	68,642	59,127
Marketable securities, Unrealized gains	68	16
Marketable securities, Unrealized losses	(2)	(142)
Marketable securities, Estimated Fair Value	68,708	59,001
Commercial paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities, Amortized cost	34,221	27,289
Marketable securities, Unrealized gains	8
Marketable securities, Unrealized losses	(3)
Marketable securities, Estimated Fair Value	$ 34,226	$ 27,289