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	Mar. 31, 2019	Dec. 31, 2018
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents, Amortized cost	$ 113,124	$ 126,313
Cash and cash equivalents, Unrealized losses	(10)	(3)
Cash and cash equivalents, Estimated Fair Value	113,114	126,310
Marketable securities, Amortized cost	153,775	151,735
Marketable securities, Unrealized gains	77	19
Marketable securities, Unrealized losses	(58)	(191)
Marketable securities, Estimated Fair Value	153,794	151,563
Cash
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents, Amortized cost	23,339	36,124
Cash and cash equivalents, Estimated Fair Value	23,339	36,124
Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents, Amortized cost	30,305	22,082
Cash and cash equivalents, Estimated Fair Value	30,305	22,082
Commercial Paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents, Amortized cost	55,790	62,413
Cash and cash equivalents, Unrealized losses	(10)
Cash and cash equivalents, Estimated Fair Value	55,780	62,413
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	2,493
Cash and cash equivalents, Estimated Fair Value	2,493
Marketable securities, Amortized cost	53,735	59,127
Marketable securities, Unrealized gains	48	16
Marketable securities, Unrealized losses	(48)	(142)
Marketable securities, Estimated Fair Value	53,735	59,001
Corporate Debt Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents, Amortized cost	1,197	5,694
Cash and cash equivalents, Unrealized losses		(3)
Cash and cash equivalents, Estimated Fair Value	1,197	5,691
Marketable securities, Amortized cost	65,450	65,319
Marketable securities, Unrealized gains	26	3
Marketable securities, Unrealized losses	(6)	(49)
Marketable securities, Estimated Fair Value	65,470	65,273
Commercial paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities, Amortized cost	34,590	27,289
Marketable securities, Unrealized gains	3
Marketable securities, Unrealized losses	(4)
Marketable securities, Estimated Fair Value	$ 34,589	$ 27,289