Fair Value Measurements - Marketable Debt Securities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Cost	$ 87,016	$ 76,162	$ 72,249
Gross Unrealized Gains	3,695	633	1,169
Gross Unrealized Losses	0	(433)	(197)
Estimated Fair Value	90,711	76,362	73,221
Other-Than-Temporary Impairments	0	0	0
Municipal bonds — available-for-sale
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Cost	87,016	76,162	72,249
Gross Unrealized Gains	3,695	633	1,169
Gross Unrealized Losses	0	(433)	(197)
Estimated Fair Value	90,711	76,362	73,221
Other-Than-Temporary Impairments	$ 0	$ 0	$ 0