Fair Value Measurements - Changes in Level 3 Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Beginning Balance	$ 164	$ 101	$ 169
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	0	(5)	5
Purchases and Issuances			0
Sales and Settlements			0
Transfers In (Out)	60	106	60
Assets Ending Balance	104	0	104
Auction rate securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Beginning Balance	72,621	21,699	87,398
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	2,163	(1)	2,218
Purchases and Issuances	(2,150)	0	(6,250)
Sales and Settlements	(34,925)	(350)	36,145
Transfers In (Out)	16,253	21,350	33,855
Assets Ending Balance	21,430	$ 0	21,430
Auction Rate Securities Purchase Commitment
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Purchases and Issuances	0		0
Sales and Settlements	0		0
Transfers In (Out)	(1,078)		(1,169)
Liabilities Beginning Balance	131		8
Total Realized and Unrealized Gains (Loss), Liabilities	(948)		(1,162)
Liabilities Ending Balance	1		1
Municipal obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Beginning Balance			35
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings			(14)
Purchases and Issuances			(76)
Sales and Settlements			125
Transfers In (Out)			0
Assets Ending Balance	$ 0		$ 0