Fair Value Measurements - Quantitative Information about Level 3 Fair Value Measurements (Details)	6 Months Ended
	Jun. 30, 2017 USD ($)
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	$ 124,188,000
Valuation Adjustment	2,309,000
Fair Value	$ 121,879,000
Auction Rate Preferred Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
U S Treasury Rate	1.72%
Student Loan Auction Rate Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Spread To U S Treasury Rate	1.20%
U S Treasury Rate	2.14%
Minimum [Member] | Auction Rate Preferred Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Spread To U S Treasury Rate	110.00%
Maximum [Member] | Auction Rate Preferred Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Spread To U S Treasury Rate	150.00%
Auction Rate Securities Owned [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	$ 109,225,000
Valuation Adjustment	2,055,000
Fair Value	107,170,000
Auction Rate Securities Owned [Member] | Auction Rate Preferred Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	108,900,000
Valuation Adjustment	2,037,000
Fair Value	$ 106,863,000
Valuation Technique	Discounted Cash Flow
Duration	4 years
Auction Rate Securities Owned [Member] | Municipal Auction Rate Securities 01 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	$ 25,000
Valuation Adjustment	3,000
Fair Value	$ 22,000
Valuation Technique	Secondary Market Trading Activity
Auction Rate Securities Owned [Member] | Student Loan Auction Rate Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	$ 300,000
Valuation Adjustment	15,000
Fair Value	$ 285,000
Valuation Technique	Discounted Cash Flow
Discount Rate	3.34%
Duration	7 years
Current Yield	2.51%
Auction Rate Securities Owned [Member] | Minimum [Member] | Auction Rate Preferred Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	1.89%
Current Yield	1.50%
Auction Rate Securities Owned [Member] | Maximum [Member] | Auction Rate Preferred Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	2.58%
Current Yield	1.93%
Auction Rate Securities Owned [Member] | Maximum [Member] | Municipal Auction Rate Securities 01 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Inputs Observable Trades In Inactive Market For In Portfolio Securities	87.75%
Auction Rate Securities Owned [Member] | Weighted Average [Member] | Auction Rate Preferred Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	2.17%
Duration	4 years
Current Yield	1.68%
Auction Rate Securities Owned [Member] | Weighted Average [Member] | Municipal Auction Rate Securities 01 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Inputs Observable Trades In Inactive Market For In Portfolio Securities	87.75%
Auction Rate Securities Owned [Member] | Weighted Average [Member] | Student Loan Auction Rate Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	3.34%
Duration	7 years
Current Yield	2.51%
Auction Rate Securities Purchase Commitment [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	$ 14,963,000
Valuation Adjustment	254,000
Fair Value	14,709,000
Auction Rate Securities Purchase Commitment [Member] | Auction Rate Preferred Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	14,937,000
Valuation Adjustment	253,000
Fair Value	$ 14,684,000
Valuation Technique	Discounted Cash Flow
Duration	4 years
Auction Rate Securities Purchase Commitment [Member] | Student Loan Auction Rate Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	3.34%
Duration	7 years
Current Yield	2.51%
Auction Rate Securities Purchase Commitment [Member] | Minimum [Member] | Auction Rate Preferred Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	1.89%
Current Yield	1.50%
Auction Rate Securities Purchase Commitment [Member] | Maximum [Member] | Auction Rate Preferred Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	2.58%
Current Yield	1.93%
Auction Rate Securities Purchase Commitment [Member] | Maximum [Member] | Municipal Auction Rate Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Inputs Observable Trades In Inactive Market For In Portfolio Securities	87.75%
Auction Rate Securities Purchase Commitment [Member] | Weighted Average [Member] | Auction Rate Preferred Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	2.17%
Duration	4 years
Current Yield	1.68%
Auction Rate Securities Purchase Commitment [Member] | Weighted Average [Member] | Municipal Auction Rate Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Inputs Observable Trades In Inactive Market For In Portfolio Securities	87.75%
Auction Rate Securities Purchase Commitment [Member] | Weighted Average [Member] | Student Loan Auction Rate Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	3.34%
Duration	7 years
Current Yield	2.51%