Summary of Significant Accounting Policies - Investments - Amortized Cost (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Amortized cost
Amortized Cost	$ 143,561	$ 101,599
Gross Unrealized Gains	14	78
Gross Unrealized Losses	(116)	(16)
Estimated Fair Value	143,459	101,661	$ 98,878
U.S. Government and agency securities
Amortized cost
Amortized Cost	42,389	30,742
Gross Unrealized Gains	11	38
Gross Unrealized Losses	(1)	(5)
Estimated Fair Value	42,399	30,775
Corporate debt securities
Amortized cost
Amortized Cost	99,861	69,552
Gross Unrealized Gains	3	40
Gross Unrealized Losses	(115)	(11)
Estimated Fair Value	99,749	69,581
Certificates of deposit
Amortized cost
Amortized Cost	1,311	1,305
Estimated Fair Value	$ 1,311	$ 1,305