Fair Value Measurements - Quantitative Information about Level 3 Fair Value Measurements (Details)	3 Months Ended
	Mar. 31, 2018 USD ($)
Auction Rate Securities Owned
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	$ 88,525,000
Valuation Adjustment	1,175,000
Fair Value	$ 87,350,000
Municipal Auction Rate Securities 01 [Member] | Auction Rate Securities Owned
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Valuation Technique	Discounted Cash Flow
Municipal Auction Rate Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Spread To U S Treasury Rate	175.00%
U S Treasury Rate	2.38%
Municipal Auction Rate Securities [Member] | Auction Rate Securities Owned
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	$ 300,000
Valuation Adjustment	10,000
Fair Value	$ 290,000
Discount Rate	4.17%
Duration	3 years
Fair Value Unobservable Input Current Yield	3.00%
Municipal Auction Rate Securities [Member] | Weighted Average | Auction Rate Securities Owned
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	4.17%
Duration	3 years
Fair Value Unobservable Input Current Yield	3.00%
Student Loan Auction Rate Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Spread To U S Treasury Rate	1.20%
U S Treasury Rate	2.59%
Student Loan Auction Rate Securities | Auction Rate Securities Owned
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	$ 275,000
Valuation Adjustment	7,000
Fair Value	$ 268,000
Valuation Technique	Discounted Cash Flow
Discount Rate	3.79%
Duration	5 years 6 months
Fair Value Unobservable Input Current Yield	3.23%
Student Loan Auction Rate Securities | Weighted Average | Auction Rate Securities Owned
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	3.79%
Duration	5 years 6 months
Fair Value Unobservable Input Current Yield	3.23%
Auction rate securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	$ 104,551,000
Valuation Adjustment For Level Three Asset and Liability	1,358,000
Fair Value Of Level Three Assets and Liabilities	$ 103,193,000
Auction Rate Preferred Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
U S Treasury Rate	2.32%
Auction Rate Preferred Securities | Auction Rate Securities Owned
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	$ 87,950,000
Valuation Adjustment	1,158,000
Fair Value	$ 86,792,000
Valuation Technique	Discounted Cash Flow
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%
Auction Rate Preferred Securities | Minimum [Member] | Auction Rate Securities Owned
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	2.56%
Fair Value Unobservable Input Current Yield	2.15%
Auction Rate Preferred Securities | Maximum [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Spread To U S Treasury Rate	150.00%
Auction Rate Preferred Securities | Maximum [Member] | Auction Rate Securities Owned
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	3.49%
Fair Value Unobservable Input Current Yield	2.79%
Auction Rate Preferred Securities | Weighted Average | Auction Rate Securities Owned
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	3.05%
Duration	2 years 6 months
Fair Value Unobservable Input Current Yield	2.49%
Auction Rate Securities Purchase Commitment
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	$ 16,026,000
Valuation Adjustment For Level Three Liabilities	183,000
Fair Value Of Level Three Liabilities	15,843,000
Auction Rate Securities Purchase Commitment | Municipal Auction Rate Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	27,000
Valuation Adjustment For Level Three Liabilities	1,000
Fair Value Of Level Three Liabilities	$ 26,000
Valuation Technique	Discounted Cash Flow
Discount Rate	4.17%
Duration	3 years
Fair Value Unobservable Input Current Yield	3.00%
Auction Rate Securities Purchase Commitment | Municipal Auction Rate Securities [Member] | Weighted Average
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	4.17%
Duration	3 years
Fair Value Unobservable Input Current Yield	3.00%
Auction Rate Securities Purchase Commitment | Student Loan Auction Rate Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	$ 25,000
Valuation Adjustment For Level Three Liabilities	1,000
Fair Value Of Level Three Liabilities	$ 24,000
Valuation Technique	Discounted Cash Flow
Discount Rate	3.79%
Duration	5 years 6 months
Fair Value Unobservable Input Current Yield	3.23%
Auction Rate Securities Purchase Commitment | Student Loan Auction Rate Securities | Weighted Average
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	3.79%
Duration	5 years 6 months
Fair Value Unobservable Input Current Yield	3.23%
Auction Rate Securities Purchase Commitment | Auction Rate Preferred Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	$ 15,974,000
Valuation Adjustment For Level Three Liabilities	181,000
Fair Value Of Level Three Liabilities	$ 15,793,000
Valuation Technique	Discounted Cash Flow
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	2.56%
Fair Value Unobservable Input Current Yield	2.15%
Auction Rate Securities Purchase Commitment | Auction Rate Preferred Securities | Maximum [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	3.49%
Fair Value Unobservable Input Current Yield	2.79%
Auction Rate Securities Purchase Commitment | Auction Rate Preferred Securities | Weighted Average
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	3.05%
Duration	2 years 6 months
Fair Value Unobservable Input Current Yield	2.49%