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, 2018	Dec. 31, 2017
Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Beginning Balance	$ 169	$ 158
Total Realized and Unrealized Gains (Losses), Assets	(8)	11
Purchases and Issuances	0
Sales and Settlements	0
Transfers In (Out)	(60)
Assets Ending Balance	101	169
Auction Rate Securities Purchase Commitment
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Beginning Balance	0	849
Total Realized and Unrealized Gains (Losses), Assets		(849)
Assets Ending Balance		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Issuances	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Purchases, (Sales), Issuances, (Settlements)	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Liability, Transfers Into Level 3	1,096
Auction rate securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Beginning Balance	87,398	84,926
Total Realized and Unrealized Gains (Losses), Assets	1,351	1,177
Purchases and Issuances	6,300	27,225
Sales and Settlements	(35,675)	(1,475)
Transfers In (Out)	(37,675)	(24,455)
Assets Ending Balance	21,699	87,398
Auction Rate Securities Purchase Commitment
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities Beginning Balance	8	645
Total Realized and Unrealized Gains (Loss), Liabilities	(1,088)	637
Liabilities Ending Balance	0	8
Municipal obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Beginning Balance	35	44
Total Realized and Unrealized Gains (Losses), Assets	14	(9)
Purchases and Issuances	76
Sales and Settlements	(125)	0
Transfers In (Out)	0
Assets Ending Balance	$ 0	$ 35