Marketable Securities - Amortized Cost and Fair Value of Debt Securities (Details) - USD ($)	Sep. 30, 2020	Dec. 31, 2019
Fair Value to Amortized Cost
Amortized Cost	$ 232,695,000
Allowance for Credit Losses	0
Net Amortized Cost, Total	232,695,000
Gross Unrealized Gains	10,000
Gross Unrealized Losses	(83,000)
Fair Value	232,622,000	$ 0
Corporate bonds - U.S.
Fair Value to Amortized Cost
Amortized Cost	136,664,000
Allowance for Credit Losses	0
Net Amortized Cost, Total	136,664,000
Gross Unrealized Gains	3,000
Gross Unrealized Losses	(69,000)
Fair Value	136,598,000
Corporate bonds - Foreign
Fair Value to Amortized Cost
Amortized Cost	16,084,000
Allowance for Credit Losses	0
Net Amortized Cost, Total	16,084,000
Gross Unrealized Gains	0
Gross Unrealized Losses	(14,000)
Fair Value	16,070,000
Government related obligations - U.S.
Fair Value to Amortized Cost
Amortized Cost	79,947,000
Allowance for Credit Losses	0
Net Amortized Cost, Total	79,947,000
Gross Unrealized Gains	7,000
Gross Unrealized Losses	0
Fair Value	$ 79,954,000