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, 2019	Dec. 31, 2018
Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Beginning Balance	$ 101	$ 169
Total Realized and Unrealized Gains (Losses), Assets	5	(8)
Purchases and Issuances	0
Sales and Settlements	0
Transfers In (Out)	(106)
Assets Ending Balance	0	101
Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Transfers In (Out)		(60)
Municipal obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Purchases and Issuances		76
Auction rate securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Beginning Balance	21,699	87,398
Total Realized and Unrealized Gains (Losses), Assets	1	1,351
Purchases and Issuances	0	6,300
Sales and Settlements	(350)	(35,675)
Transfers In (Out)	(21,350)	(37,675)
Assets Ending Balance	0	21,699
Auction Rate Securities Purchase Commitment
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Transfers In (Out)		1,096
Liabilities Beginning Balance	0	8
Total Realized and Unrealized Gains (Loss), Liabilities		(1,088)
Liabilities Ending Balance		0
Municipal obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Beginning Balance	$ 0	35
Total Realized and Unrealized Gains (Losses), Assets		14
Sales and Settlements		(125)
Assets Ending Balance		$ 0