Short-Term Investments - Summary of Investments Included in Cash Equivalents and Short Term Investments (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Schedule Of Cash Cash Equivalents And Available For Sale Securities [Line Items]
Cash equivalents and short-term investments, Amortized Cost	$ 214,343	$ 122,339
Cash equivalents and short-term investments, Unrealized Losses	(77)	(73)
Cash equivalents and short-term investments, Fair Value	214,266	122,266
Money Market Mutual Funds
Schedule Of Cash Cash Equivalents And Available For Sale Securities [Line Items]
Cash equivalents and short-term investments, Amortized Cost	31,740	44,182
Cash equivalents and short-term investments, Fair Value	31,740	44,182
Asset-backed Securities
Schedule Of Cash Cash Equivalents And Available For Sale Securities [Line Items]
Cash equivalents and short-term investments, Amortized Cost	19,541	7,428
Cash equivalents and short-term investments, Unrealized Losses	(21)	(6)
Cash equivalents and short-term investments, Fair Value	19,520	7,422
Commercial Paper
Schedule Of Cash Cash Equivalents And Available For Sale Securities [Line Items]
Cash equivalents and short-term investments, Amortized Cost	78,571	34,882
Cash equivalents and short-term investments, Fair Value	78,571	34,882
Corporate Debt Securities
Schedule Of Cash Cash Equivalents And Available For Sale Securities [Line Items]
Cash equivalents and short-term investments, Amortized Cost	54,511	26,906
Cash equivalents and short-term investments, Unrealized Losses	(50)	(50)
Cash equivalents and short-term investments, Fair Value	54,461	26,856
U.S Treasury Securities
Schedule Of Cash Cash Equivalents And Available For Sale Securities [Line Items]
Cash equivalents and short-term investments, Amortized Cost	29,980	8,941
Cash equivalents and short-term investments, Unrealized Losses	(6)	(17)
Cash equivalents and short-term investments, Fair Value	$ 29,974	$ 8,924