Marketable Securities - Amortized Cost and Fair Value of Debt Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Fair Value to Amortized Cost
Amortized Cost	$ 101,590	$ 223,310
Allowance for Credit Losses	0	0
Net Amortized Cost, Total	101,590	223,310
Gross Unrealized Gains	4	2
Gross Unrealized Losses	(47)	(127)
Fair Value	101,547	223,185
Corporate bonds - U.S.
Fair Value to Amortized Cost
Amortized Cost	66,419	185,989
Allowance for Credit Losses	0	0
Net Amortized Cost, Total	66,419	185,989
Gross Unrealized Gains	4	1
Gross Unrealized Losses	(29)	(106)
Fair Value	66,394	185,884
Corporate bonds - Foreign
Fair Value to Amortized Cost
Amortized Cost	35,171	37,321
Allowance for Credit Losses	0	0
Net Amortized Cost, Total	35,171	37,321
Gross Unrealized Gains	0	1
Gross Unrealized Losses	(18)	(21)
Fair Value	$ 35,153	$ 37,301