Fair Value Measurements - Quantitative Information about Level 3 Fair Value Measurements (Details)	3 Months Ended	6 Months Ended
	Jun. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Level Three Principal Amount	$ 104,462,000	$ 104,462,000
Valuation Adjustment For Level Three Asset and Liability	2,299,000	2,299,000
Fair Value Of Level Three Assets and Liabilities	102,163,000	102,163,000
Auction rate securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Level Three Principal Amount	92,300,000	92,300,000
Valuation Adjustment	2,077,000	2,077,000
Fair Value	90,223,000	90,223,000
Auction Rate Preferred Securities 01 [Member] | Auction rate securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Level Three Principal Amount	18,725,000	18,725,000
Valuation Adjustment	1,123,000	1,123,000
Fair Value	17,602,000	$ 17,602,000
Valuation Technique		Tender Offer (4)
Municipal Auction Rate Securities [Member] | Auction rate securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Level Three Principal Amount	25,000	$ 25,000
Valuation Adjustment	0	0
Fair Value	$ 25,000	$ 25,000
Valuation Technique		Par
Student Loan Auction Rate Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Spread To U S Treasury Rate	1.20%	1.20%
U S Treasury Rate	2.76%	2.76%
Student Loan Auction Rate Securities | Auction rate securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Level Three Principal Amount	$ 275,000	$ 275,000
Valuation Adjustment	13,000	13,000
Fair Value	$ 262,000	$ 262,000
Valuation Technique		Discounted Cash Flow
Discount Rate		3.96%
Fair Value Unobservable Inputs Duration		5 years 6 months
Fair Value Unobservable Input Current Yield	2.95%	2.95%
Student Loan Auction Rate Securities | Weighted Average [Member] | Auction rate securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate		3.96%
Fair Value Unobservable Inputs Duration		5 years 6 months
Fair Value Unobservable Input Current Yield	2.95%	2.95%
Auction Rate Preferred Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
U S Treasury Rate	2.58%	2.58%
Auction Rate Preferred Securities | Auction rate securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Level Three Principal Amount	$ 73,275,000	$ 73,275,000
Valuation Adjustment	941,000	941,000
Fair Value	$ 72,334,000	$ 72,334,000
Valuation Technique		Discounted Cash Flow
Fair Value Unobservable Inputs Duration		2 years 6 months
Auction Rate Preferred Securities | Minimum [Member]
Fair Value Measurements, Recurring and Nonrecurring, 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 Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate		2.83%
Fair Value Unobservable Input Current Yield	2.36%	2.36%
Auction Rate Preferred Securities | Maximum [Member]
Fair Value Measurements, Recurring and Nonrecurring, 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 Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate		3.86%
Fair Value Unobservable Input Current Yield	3.21%	3.21%
Auction Rate Preferred Securities | Weighted Average [Member] | Auction rate securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate		3.22%
Fair Value Unobservable Inputs Duration		2 years 6 months
Fair Value Unobservable Input Current Yield	2.68%	2.68%
Fair Value, Valuation Scenario One [Member] | Auction rate securities
Fair Value Measurements, Recurring and Nonrecurring, 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	$ 493,000	$ 493,000
Fair Value, Valuation Scenario Two [Member] | Auction rate securities
Fair Value Measurements, Recurring and Nonrecurring, 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	$ 982,000	$ 982,000
Auction Rate Securities Purchase Commitment
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Transfers, Net	91,000
Level Three Principal Amount	12,162,000	12,162,000
Valuation Adjustment For Level Three Liabilities	222,000	222,000
Fair Value Of Level Three Liabilities	11,940,000	11,940,000
Auction Rate Securities Purchase Commitment | Auction Rate Preferred Securities 01 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Level Three Principal Amount	1,515,000	1,515,000
Valuation Adjustment For Level Three Liabilities	91,000	91,000
Fair Value Of Level Three Liabilities	1,424,000	$ 1,424,000
Valuation Technique		Tender Offer (4)
Auction Rate Securities Purchase Commitment | Municipal Auction Rate Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Level Three Principal Amount	2,000	$ 2,000
Valuation Adjustment For Level Three Liabilities	0	0
Fair Value Of Level Three Liabilities	2,000	$ 2,000
Valuation Technique		Par
Auction Rate Securities Purchase Commitment | Student Loan Auction Rate Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Level Three Principal Amount	25,000	$ 25,000
Valuation Adjustment For Level Three Liabilities	1,000	1,000
Fair Value Of Level Three Liabilities	$ 24,000	$ 24,000
Valuation Technique		Discounted Cash Flow
Discount Rate		3.96%
Fair Value Unobservable Inputs Duration		5 years 6 months
Fair Value Unobservable Input Current Yield	2.95%	2.95%
Auction Rate Securities Purchase Commitment | Student Loan Auction Rate Securities | Weighted Average [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate		3.96%
Fair Value Unobservable Inputs Duration		5 years 6 months
Fair Value Unobservable Input Current Yield	2.95%	2.95%
Auction Rate Securities Purchase Commitment | Auction Rate Preferred Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Level Three Principal Amount	$ 10,620,000	$ 10,620,000
Valuation Adjustment For Level Three Liabilities	130,000	130,000
Fair Value Of Level Three Liabilities	$ 10,490,000	$ 10,490,000
Valuation Technique		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 Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate		2.83%
Fair Value Unobservable Input Current Yield	2.36%	2.36%
Auction Rate Securities Purchase Commitment | Auction Rate Preferred Securities | Maximum [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate		3.86%
Fair Value Unobservable Input Current Yield	3.21%	3.21%
Auction Rate Securities Purchase Commitment | Auction Rate Preferred Securities | Weighted Average [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate		3.22%
Fair Value Unobservable Inputs Duration		2 years 6 months
Fair Value Unobservable Input Current Yield	2.68%	2.68%