Fair Value Measurements - Changes in Level 3 Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
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				$ (10,000)
Assets Beginning Balance				158,000
Purchases and Issuances				0
Sales and Settlements				0
Transfers In (Out)				0
Assets Ending Balance		$ 168,000		168,000
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	$ 4,000	(4,000)
Assets Beginning Balance	168,000	164,000
Assets Ending Balance	164,000	168,000	$ 164,000	168,000
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			5,000
Assets Beginning Balance			169,000
Purchases and Issuances			0
Sales and Settlements			0
Transfers In (Out)			0
Assets Ending Balance	164,000		164,000
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		0	(14,000)	8,000
Assets Beginning Balance		36,000	35,000	44,000
Purchases and Issuances			(76,000)	0
Sales and Settlements			(125,000)	0
Transfers In (Out)			0	0
Assets Ending Balance	0	36,000	0	36,000
ARS purchase commitments
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		878,000		849,000
Assets Beginning Balance		878,000		849,000
Purchases and Issuances				0
Sales and Settlements				0
Transfers In (Out)				0
Assets Ending Balance		0		0
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	902,000	(502,000)	55,000	1,144,000
Assets Beginning Balance	87,350,000	89,743,000	87,398,000	(84,926,000)
Purchases and Issuances	(4,050,000)	(17,050,000)	(4,100,000)	22,050,000
Sales and Settlements	(275,000)	(125,000)	(1,220,000)	(950,000)
Transfers In (Out)	17,602,000		17,602,000	0
Assets Ending Balance	72,621,000	107,170,000	72,621,000	107,170,000
ARS purchase commitments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Purchases and Issuances			0
Sales and Settlements			0
Transfers In (Out)			(91,000)
Liabilities Beginning Balance	183,000	359,000	8,000	645,000
Total Realized and Unrealized Gains (Loss), Liabilities	(39,000)	105,000	(214,000)	391,000
Liabilities Ending Balance	131,000	254,000	$ 131,000	254,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Transfers, Net	91,000
Auction rate securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Transfers, Net	$ 17,600,000	$ 0		$ 0