Available for Sale Securities - Summary of Available for Sale Securities at Fair Value (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 105,333	$ 217,257
Gross Unrealized Gains	22	1
Gross Unrealized Losses	(58)	(262)
Fair Value	105,297	216,996
Cash and cash equivalents	62,741	104,644	$ 158,171
Cash and cash equivalents, Fair Value	62,741	104,644
Total cash, cash equivalents, and available for sale securities - Amortized Cost	168,074	321,901
Total cash, cash equivalents, and available for sale securities - Fair Value	168,038	321,640
U.S. government debt securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	89,549	158,518
Gross Unrealized Gains	22	1
Gross Unrealized Losses	(53)	(198)
Fair Value	89,518	158,321
Certificates of deposit
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	245	245
Gross Unrealized Losses	(1)
Fair Value	244	245
Corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	15,539	58,494
Gross Unrealized Losses	(4)	(64)
Fair Value	$ 15,535	$ 58,430

Cash cash equivalents and short term investments amortized cost.

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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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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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