Fair Value Measurements - Changes in Level 3 Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
ARS purchase commitments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Beginning Balance	$ 0
Total Realized and Unrealized Gains (Losses), Assets	(137)
Purchases and Issuances	0
Sales and Settlements	0
Transfers In (Out)	332
Assets Ending Balance	195	$ 0
Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Beginning Balance	0	101
Total Realized and Unrealized Gains (Losses), Assets		5
Transfers In (Out)		(106)
Assets Ending Balance		0
Auction rate securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Beginning Balance	0	21,699
Total Realized and Unrealized Gains (Losses), Assets	(165)	1
Purchases and Issuances	1,300	0
Sales and Settlements	0	(350)
Transfers In (Out)	29,566	(21,350)
Assets Ending Balance	$ 30,701	$ 0