Fair Value Measurements - Quantitative Information about Level 3 Fair Value Measurements (Details)	12 Months Ended
	Dec. 31, 2018 USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Level Two and Three Principal Amount	$ 47,955,000
Valuation Adjustment For Level Two and Three Assets	3,794,000
Fair Value Of Level Two and Three Assets	$ 44,161,000
Auction Rate Preferred Securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
U S Treasury Rate	2.60%
Municipal Auction Rate Securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Spread To U S Treasury Rate	175.00%
U S Treasury Rate	2.49%
Student Loan Auction Rate Securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Spread To U S Treasury Rate	1.20%
U S Treasury Rate	2.48%
Auction Rate Securities Owned
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Level Two and Three Principal Amount	$ 40,650,000
Valuation Adjustment For Level Two and Three Assets	2,698,000
Fair Value Of Level Two and Three Assets	37,952,000
Auction Rate Securities Owned | Auction Rate Preferred Securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Principal	21,350,000
Valuation Adjustment	1,000
Fair Value	$ 21,349,000
Fair Value Measurement, Valuation Techniques	Discounted Cash Flow
Weighted Average Discount Rate, Percent	3.88%
Duration	1 year
Auction Rate Securities Owned | Auction Rate Preferred Securities 02 [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Measurement, Valuation Techniques	Tender Offer (4)
Level Two Principal Amount	$ 18,950,000
Valuation Adjustment For Level Two Assets	2,697,000
Fair Value Of Level Two Assets	16,253,000
Auction Rate Securities Owned | Municipal Auction Rate Securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Principal	75,000
Valuation Adjustment	0
Fair Value	$ 75,000
Fair Value Measurement, Valuation Techniques	Discounted Cash Flow
Fair Value Input, Discount Rate	4.35%
Weighted Average Discount Rate, Percent	4.35%
Duration	2 years
Fair Value Unobservable Input Current Yield	5.51%
Auction Rate Securities Owned | Student Loan Auction Rate Securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Principal	$ 275,000
Valuation Adjustment	0
Fair Value	$ 275,000
Fair Value Measurement, Valuation Techniques	Discounted Cash Flow
Fair Value Input, Discount Rate	3.68%
Weighted Average Discount Rate, Percent	3.68%
Duration	4 years
Fair Value Unobservable Input Current Yield	3.64%
Minimum [Member] | Auction Rate Preferred Securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Spread To U S Treasury Rate	110.00%
Minimum [Member] | Auction Rate Securities Owned | Auction Rate Preferred Securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Input, Discount Rate	2.86%
Fair Value Unobservable Input Current Yield	2.69%
Maximum [Member] | Auction Rate Preferred Securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Spread To U S Treasury Rate	150.00%
Maximum [Member] | Auction Rate Securities Owned | Auction Rate Preferred Securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Input, Discount Rate	3.89%
Maximum [Member] | ARS purchase commitments | Auction Rate Preferred Securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Unobservable Input Current Yield	4.05%
Weighted Average | Auction Rate Securities Owned | Auction Rate Preferred Securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Duration	1 year
Fair Value Unobservable Input Current Yield	4.03%
Weighted Average | Auction Rate Securities Owned | Municipal Auction Rate Securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Unobservable Input Current Yield	5.51%
Weighted Average | Auction Rate Securities Owned | Student Loan Auction Rate Securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Duration	4 years
Fair Value Unobservable Input Current Yield	3.64%
Weighted Average | Municipal Auction Rate Securities | Auction Rate Preferred Securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Duration	2 years
ARS purchase commitments
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Measurement, Valuation Techniques	Tender Offer (4)
Level Two Principal Amount	$ 7,305,000
Valuation Adjustment For Level Two Assets	1,096,000
Fair Value Of Level Two Assets	$ 6,209,000
Other Assets | Minimum [Member] | ARS purchase commitments | Auction Rate Preferred Securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Input, Discount Rate	2.86%
Fair Value Unobservable Input Current Yield	2.69%
Other Assets | Maximum [Member] | ARS purchase commitments | Auction Rate Preferred Securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Input, Discount Rate	3.89%
Fair Value Unobservable Input Current Yield	4.05%