Fair Value Measurements - Changes in Level 3 Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Beginning Balance	$ 101	$ 169
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	(3)	1
Purchases and Issuances		0
Sales and Settlements		0
Transfers In (Out)	0	0
Assets Ending Balance	104	168
Auction rate securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Beginning Balance	21,699	87,398
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	(1)	(847)
Purchases and Issuances	0	(50)
Sales and Settlements	0	(945)
Transfers In (Out)	0	0
Assets Ending Balance	$ 21,700	87,350
Auction Rate Securities Purchase Commitment
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Purchases and Issuances		0
Sales and Settlements		0
Transfers In (Out)		0
Liabilities Beginning Balance		8
Total Realized and Unrealized Gains (Loss), Liabilities		(175)
Liabilities Ending Balance		183
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