Fair Value Measurements - Marketable Debt Securities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Amortized Cost	$ 117,970	$ 87,016	$ 76,162
Gross Unrealized Gains	7,043	3,695	633
Gross Unrealized Losses	(1)	0	(433)
Estimated Fair Value	125,012	90,711	76,362
Credit Loss Impairments	0	0	0
Municipal bonds — available-for-sale
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Amortized Cost	117,970	87,016	76,162
Gross Unrealized Gains	7,043	3,695	633
Gross Unrealized Losses	(1)	0	(433)
Estimated Fair Value	125,012	90,711	76,362
Credit Loss Impairments	$ 0	$ 0	$ 0