Fair Value Measurements - Quantitative Information about Level 3 Fair Value Measurements (Details)	3 Months Ended
	Mar. 31, 2016 USD ($)
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	$ 119,674,000
Valuation Adjustment	1,959,000
Fair Value	$ 117,715,000
Auction Rate Preferred Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
U S Treasury Rate	1.04%
Municipal Auction Rate Securities 01 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Spread To U S Treasury Rate	175.00%
U S Treasury Rate	1.13%
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	1.55%
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	$ 87,125,000
Valuation Adjustment	2,940,000
Fair Value	84,185,000
Auction Rate Securities Owned [Member] | Auction Rate Preferred Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	83,175,000
Valuation Adjustment	2,406,000
Fair Value	$ 80,769,000
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	1,000
Fair Value	$ 24,000
Valuation Technique	Discounted Cash Flow
Discount Rate	1.98%
Duration	4 years 6 months
Current Yield	1.15%
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	25,000
Fair Value	$ 275,000
Valuation Technique	Discounted Cash Flow
Discount Rate	2.75%
Duration	7 years
Current Yield	1.44%
Auction Rate Securities Owned [Member] | Other Auction Rate Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	$ 3,625,000
Valuation Adjustment	508,000
Fair Value	$ 3,117,000
Valuation Technique	Secondary Market Trading Activity
Percent of Par	85.99%
Auction Rate Securities Owned [Member] | Minimum [Member] | Auction Rate Preferred Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	1.15%
Current Yield	0.47%
Auction Rate Securities Owned [Member] | Maximum [Member] | Auction Rate Preferred Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	1.56%
Current Yield	0.75%
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	1.36%
Duration	4 years
Current Yield	0.61%
Auction Rate Securities Owned [Member] | Weighted Average [Member] | Municipal Auction Rate Securities 01 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	1.98%
Duration	4 years 6 months
Current Yield	1.15%
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	2.75%
Duration	7 years
Current Yield	1.44%
Auction Rate Securities Owned [Member] | Weighted Average [Member] | Other Auction Rate Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Percent of Par	85.99%
Auction Rate Securities Purchase Commitment [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	$ 32,549,000
Valuation Adjustment	(981,000)
Fair Value	33,530,000
Auction Rate Securities Purchase Commitment [Member] | Auction Rate Preferred Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	11,237,000
Valuation Adjustment	(1,540,000)
Fair Value	$ 12,777,000
Valuation Technique	Discounted Cash Flow
Duration	4 years
Auction Rate Securities Purchase Commitment [Member] | Auction Rate Preferred Securities 01 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	$ 21,312,000
Valuation Adjustment	559,000
Fair Value	$ 20,753,000
Valuation Technique	Discounted Cash Flow
Duration	4 years
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	1.15%
Current Yield	0.47%
Auction Rate Securities Purchase Commitment [Member] | Minimum [Member] | Auction Rate Preferred Securities 01 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	1.15%
Current Yield	0.47%
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	1.56%
Current Yield	0.75%
Auction Rate Securities Purchase Commitment [Member] | Maximum [Member] | Auction Rate Preferred Securities 01 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	1.56%
Current Yield	0.75%
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	1.36%
Duration	4 years
Current Yield	0.61%
Auction Rate Securities Purchase Commitment [Member] | Weighted Average [Member] | Auction Rate Preferred Securities 01 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	1.36%
Duration	4 years
Current Yield	0.61%