Short-Term Investments - Summary of Investments Included in Cash Equivalents and Short Term Investments (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Schedule Of Cash Cash Equivalents And Available For Sale Securities [Line Items]
Cash equivalents and short-term investments, Amortized Cost	$ 295,126	$ 214,343
Cash equivalents and short-term investments, Unrealized Gains	186
Cash equivalents and short-term investments, Unrealized Losses		(77)
Cash equivalents and short-term investments, Fair Value	295,312	214,266
Money Market Mutual Funds
Schedule Of Cash Cash Equivalents And Available For Sale Securities [Line Items]
Cash equivalents and short-term investments, Amortized Cost	54,989	31,740
Cash equivalents and short-term investments, Fair Value	54,989	31,740
Repurchase Agreements
Schedule Of Cash Cash Equivalents And Available For Sale Securities [Line Items]
Cash equivalents and short-term investments, Amortized Cost	60,000
Cash equivalents and short-term investments, Fair Value	60,000
Asset-backed Securities
Schedule Of Cash Cash Equivalents And Available For Sale Securities [Line Items]
Cash equivalents and short-term investments, Amortized Cost	16,968	19,541
Cash equivalents and short-term investments, Unrealized Gains	11
Cash equivalents and short-term investments, Unrealized Losses		(21)
Cash equivalents and short-term investments, Fair Value	16,979	19,520
Commercial Paper
Schedule Of Cash Cash Equivalents And Available For Sale Securities [Line Items]
Cash equivalents and short-term investments, Amortized Cost	51,410	78,571
Cash equivalents and short-term investments, Fair Value	51,410	78,571
Corporate Debt Securities
Schedule Of Cash Cash Equivalents And Available For Sale Securities [Line Items]
Cash equivalents and short-term investments, Amortized Cost	79,766	54,511
Cash equivalents and short-term investments, Unrealized Gains	146
Cash equivalents and short-term investments, Unrealized Losses		(50)
Cash equivalents and short-term investments, Fair Value	79,912	54,461
U.S Treasury Securities
Schedule Of Cash Cash Equivalents And Available For Sale Securities [Line Items]
Cash equivalents and short-term investments, Amortized Cost	31,993	29,980
Cash equivalents and short-term investments, Unrealized Gains	29
Cash equivalents and short-term investments, Unrealized Losses		(6)
Cash equivalents and short-term investments, Fair Value	$ 32,022	$ 29,974