Fair Value Measurements - Changes in Level 3 Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Beginning Balance	$ 104	$ 168	$ 101	$ 169
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	(2)	4	(5)	5
Purchases and Issuances				0
Sales and Settlements				0
Transfers In (Out)	106	0	106	0
Assets Ending Balance	0	164	0	164
Auction rate securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Beginning Balance	21,700	87,350	21,699	87,398
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	0	902	(1)	55
Purchases and Issuances	0	(4,050)	0	(4,100)
Sales and Settlements	(350)	(275)	(350)	1,220
Transfers In (Out)	21,350	17,602	21,350	17,602
Assets Ending Balance	$ 0	72,621	$ 0	72,621
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)		(91)		(91)
Liabilities Beginning Balance		183		8
Total Realized and Unrealized Gains (Loss), Liabilities		(39)		(214)
Liabilities Ending Balance		131		131
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