Short-Term Investments - Summary of Investments Included in Cash Equivalents and Short Term Investments (Details) - USD ($) $ in Thousands	Sep. 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	$ 269,951	$ 214,343
Cash equivalents and short-term investments, Unrealized Gains	258
Cash equivalents and short-term investments, Unrealized Losses	(4)	(77)
Cash equivalents and short-term investments, Fair Value	270,205	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	36,883	31,740
Cash equivalents and short-term investments, Fair Value	36,883	31,740
Repurchase Agreements
Schedule Of Cash Cash Equivalents And Available For Sale Securities [Line Items]
Cash equivalents and short-term investments, Amortized Cost	41,200
Cash equivalents and short-term investments, Fair Value	41,200
Asset-backed Securities
Schedule Of Cash Cash Equivalents And Available For Sale Securities [Line Items]
Cash equivalents and short-term investments, Amortized Cost	11,582	19,541
Cash equivalents and short-term investments, Unrealized Gains	12
Cash equivalents and short-term investments, Unrealized Losses		(21)
Cash equivalents and short-term investments, Fair Value	11,594	19,520
Commercial Paper
Schedule Of Cash Cash Equivalents And Available For Sale Securities [Line Items]
Cash equivalents and short-term investments, Amortized Cost	23,074	78,571
Cash equivalents and short-term investments, Fair Value	23,074	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	92,418	54,511
Cash equivalents and short-term investments, Unrealized Gains	210
Cash equivalents and short-term investments, Unrealized Losses		(50)
Cash equivalents and short-term investments, Fair Value	92,628	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	64,794	29,980
Cash equivalents and short-term investments, Unrealized Gains	36
Cash equivalents and short-term investments, Unrealized Losses	(4)	(6)
Cash equivalents and short-term investments, Fair Value	$ 64,826	$ 29,974