Fair Value Measurements - Quantitative Information about Level 3 Fair Value Measurements (Details)	3 Months Ended	9 Months Ended
	Sep. 30, 2018 USD ($)	Sep. 30, 2018 USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Level Three Principal Amount	$ 48,033,000	$ 48,033,000
Valuation Adjustment For Level Three Asset and Liability	4,196,000	4,196,000
Fair Value Of Level Three Assets and Liabilities	43,837,000	43,837,000
Auction rate securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Level Three Principal Amount	40,800,000	40,800,000
Valuation Adjustment	3,117,000	3,117,000
Fair Value	37,683,000	37,683,000
Auction Rate Preferred Securities 01 [Member] | Auction rate securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Level Three Principal Amount	18,950,000	18,950,000
Valuation Adjustment	2,697,000	2,697,000
Fair Value	16,253,000	$ 16,253,000
Fair Value Measurement, Valuation Techniques		Tender Offer (4)
Municipal Auction Rate Securities [Member] | Auction rate securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Level Three Principal Amount	75,000	$ 75,000
Valuation Adjustment	0	0
Fair Value	$ 75,000	$ 75,000
Fair Value Measurement, Valuation Techniques		Par
Student Loan Auction Rate Securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Spread To U S Treasury Rate	1.20%	1.20%
U S Treasury Rate	2.97%	2.97%
Student Loan Auction Rate Securities | Auction rate securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Level Three Principal Amount	$ 275,000	$ 275,000
Valuation Adjustment	14,000	14,000
Fair Value	$ 261,000	$ 261,000
Fair Value Measurement, Valuation Techniques		Discounted Cash Flow
Fair Value Input, Discount Rate		4.17%
Fair Value Unobservable Inputs Duration		5 years 6 months
Fair Value Unobservable Input Current Yield	3.09%	3.09%
Student Loan Auction Rate Securities | Weighted Average [Member] | Auction rate securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Input, Discount Rate		4.17%
Fair Value Unobservable Inputs Duration		5 years 6 months
Fair Value Unobservable Input Current Yield	3.09%	3.09%
Auction Rate Preferred Securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
U S Treasury Rate	2.85%	2.85%
Auction Rate Preferred Securities | Auction rate securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Level Three Principal Amount	$ 21,500,000	$ 21,500,000
Valuation Adjustment	406,000	406,000
Fair Value	$ 21,094,000	$ 21,094,000
Fair Value Measurement, Valuation Techniques		Discounted Cash Flow
Fair Value Unobservable Inputs Duration		2 years 6 months
Auction Rate Preferred Securities | Minimum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Spread To U S Treasury Rate	110.00%	110.00%
Auction Rate Preferred Securities | Minimum [Member] | Auction rate securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Input, Discount Rate		3.14%
Fair Value Unobservable Input Current Yield	2.38%	2.38%
Auction Rate Preferred Securities | Maximum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Spread To U S Treasury Rate	150.00%	150.00%
Auction Rate Preferred Securities | Maximum [Member] | Auction rate securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Input, Discount Rate		4.28%
Fair Value Unobservable Input Current Yield	3.46%	3.46%
Auction Rate Preferred Securities | Weighted Average [Member] | Auction rate securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Input, Discount Rate		4.26%
Fair Value Unobservable Inputs Duration		2 years 6 months
Fair Value Unobservable Input Current Yield	3.45%	3.45%
Fair Value, Valuation Scenario One [Member] | Auction rate securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Sensitivity Analysis Of Fair Value Of Interests Continued To Be Held By Transferor Servicing Assets Or Liabilities Change In Discount Rate	25.00%	25.00%
Sensitivity Analysis of Fair Value, Transferor's Interests in Transferred Financial Assets, Impact of Other than 10 or 20 Percent Adverse Change in Discount Rate	$ 126,000	$ 126,000
Fair Value, Valuation Scenario Two [Member] | Auction rate securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Sensitivity Analysis Of Fair Value Of Interests Continued To Be Held By Transferor Servicing Assets Or Liabilities Change In Discount Rate	50.00%	50.00%
Sensitivity Analysis of Fair Value, Transferor's Interests in Transferred Financial Assets, Impact of Other than 10 or 20 Percent Adverse Change in Discount Rate	$ 251,000	$ 251,000
Auction Rate Securities Purchase Commitment
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Transfers, Net	1,078,000
Level Three Principal Amount	7,233,000	7,233,000
Valuation Adjustment For Level Three Liabilities	1,079,000	1,079,000
Fair Value Of Level Three Liabilities	6,154,000	6,154,000
Auction Rate Securities Purchase Commitment | Auction Rate Preferred Securities 01 [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Level Three Principal Amount	7,184,000	7,184,000
Valuation Adjustment For Level Three Liabilities	1,078,000	1,078,000
Fair Value Of Level Three Liabilities	6,106,000	$ 6,106,000
Fair Value Measurement, Valuation Techniques		Tender Offer (4)
Auction Rate Securities Purchase Commitment | Auction Rate Preferred Securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Level Three Principal Amount	49,000	$ 49,000
Valuation Adjustment For Level Three Liabilities	1,000	1,000
Fair Value Of Level Three Liabilities	$ 48,000	$ 48,000
Fair Value Measurement, Valuation Techniques		Discounted Cash Flow
Fair Value Unobservable Inputs Duration		2 years 6 months
Auction Rate Securities Purchase Commitment | Auction Rate Preferred Securities | Minimum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Input, Discount Rate		3.14%
Fair Value Unobservable Input Current Yield	2.38%	2.38%
Auction Rate Securities Purchase Commitment | Auction Rate Preferred Securities | Maximum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Input, Discount Rate		4.28%
Fair Value Unobservable Input Current Yield	3.46%	3.46%
Auction Rate Securities Purchase Commitment | Auction Rate Preferred Securities | Weighted Average [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Input, Discount Rate		4.26%
Fair Value Unobservable Inputs Duration		2 years 6 months
Fair Value Unobservable Input Current Yield	3.45%	3.45%