Short-Term Investments - Summary of Investments Included in Cash Equivalents and Short Term Investments (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Schedule Of Cash Cash Equivalents And Available For Sale Securities [Line Items]
Cash equivalents and short-term investments, Amortized Cost	$ 187,482	$ 214,343
Cash equivalents and short-term investments, Unrealized Gains	16
Cash equivalents and short-term investments, Unrealized Losses	(10)	(77)
Cash equivalents and short-term investments, Fair Value	187,488	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	40,291	31,740
Cash equivalents and short-term investments, Fair Value	40,291	31,740
Asset-backed Securities
Schedule Of Cash Cash Equivalents And Available For Sale Securities [Line Items]
Cash equivalents and short-term investments, Amortized Cost	18,078	19,541
Cash equivalents and short-term investments, Unrealized Gains	2
Cash equivalents and short-term investments, Unrealized Losses	(1)	(21)
Cash equivalents and short-term investments, Fair Value	18,079	19,520
Commercial Paper
Schedule Of Cash Cash Equivalents And Available For Sale Securities [Line Items]
Cash equivalents and short-term investments, Amortized Cost	56,350	78,571
Cash equivalents and short-term investments, Fair Value	56,350	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	45,887	54,511
Cash equivalents and short-term investments, Unrealized Gains	6
Cash equivalents and short-term investments, Unrealized Losses	(9)	(50)
Cash equivalents and short-term investments, Fair Value	45,884	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	26,876	29,980
Cash equivalents and short-term investments, Unrealized Gains	8
Cash equivalents and short-term investments, Unrealized Losses		(6)
Cash equivalents and short-term investments, Fair Value	$ 26,884	$ 29,974

Schedule of cash, cash equivalents and available-for-sale securities.

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Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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