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, 2018	Mar. 31, 2017
Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	$ (1)	$ 6
Assets Beginning Balance	169	158
Transfers In (Out)	0
Assets Ending Balance	168	164
Municipal obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	14	(8)
Assets Beginning Balance	35	44
Purchases and Issuances	76
Sales and Settlements	(125)
Transfers In (Out)	0
Assets Ending Balance	0	36
Auction Rate Securities Purchase Commitment
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings		29
Assets Beginning Balance		849
Assets Ending Balance		878
Liabilities Beginning Balance	8	645
Total Realized and Unrealized Gains (Loss), Liabilities	(175)	286
Liabilities Ending Balance	183	359
Auction rate securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	847	642
Assets Beginning Balance	87,398	84,926
Purchases and Issuances	50	5,000
Sales and Settlements	(945)	(825)
Transfers In (Out)	0
Assets Ending Balance	$ 87,350	$ 89,743