Fair Value Measurements - Quantitative Information about Level 3 Fair Value Measurements (Details) (USD $)	3 Months Ended
	Mar. 31, 2015
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	119,500,000
Valuation Adjustment	6,990,000
Fair Value	112,510,000
Auction Rate Preferred Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
U S Treasury Rate	1.13%
Municipal Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Spread To U S Treasury Rate	175.00%
U S Treasury Rate	1.25%
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	1.71%
Hedge Funds [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Investment Redemption Notice Period Minimum	30 days
Investment Redemption Notice Period Maximum	120 days
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	105,250,000
Valuation Adjustment	6,193,000
Fair Value	99,057,000
Auction Rate Securities Owned [Member] | Auction Rate Preferred Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	83,700,000
Valuation Adjustment	3,783,000
Fair Value	79,917,000
Valuation Technique	Discounted Cash Flow
Auction Rate Securities Owned [Member] | Municipal Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	11,575,000
Valuation Adjustment	972,000
Fair Value	10,603,000
Valuation Technique	Discounted Cash Flow
Discount Rate	2.18%
Duration	4 years 6 months
Current Yield	0.21%
Auction Rate Securities Owned [Member] | Municipal Auction Rate Securities 02 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	5,975,000
Valuation Adjustment	538,000
Fair Value	5,437,000
Valuation Technique	Secondary Market Trading Activity
Observable trades in inactive market for in portfolio securities	91.00%
Auction Rate Securities Owned [Member] | Student Loan Auction Rate Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	375,000
Valuation Adjustment	47,000
Fair Value	328,000
Valuation Technique	Discounted Cash Flow
Discount Rate	2.91%
Duration	7 years
Current Yield	0.79%
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	853,000
Fair Value	2,772,000
Valuation Technique	Secondary Market Trading Activity
Observable trades in inactive market for in portfolio securities	76.46%
Auction Rate Securities Owned [Member] | Minimum [Member] | Auction Rate Preferred Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	1.24%
Duration	4 years
Current Yield	0.10%
Auction Rate Securities Owned [Member] | Maximum [Member] | Auction Rate Preferred Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	1.69%
Current Yield	0.48%
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	1.45%
Duration	4 years
Current Yield	0.28%
Auction Rate Securities Owned [Member] | Weighted Average [Member] | Municipal Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	2.18%
Duration	4 years 6 months
Current Yield	0.21%
Auction Rate Securities Owned [Member] | Weighted Average [Member] | Municipal Auction Rate Securities 02 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Observable trades in inactive market for in portfolio securities	91.00%
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	2.91%
Duration	7 years
Current Yield	0.79%
Auction Rate Securities Owned [Member] | Weighted Average [Member] | Other Auction Rate Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Observable trades in inactive market for in portfolio securities	76.46%
Auction Rate Securities Purchase Commitment [Member] | Auction Rate Preferred Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
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	2.91%
Duration	7 years
Current Yield	0.79%
Auction Rate Securities Purchase Commitment [Member] | Municipal Auction Rate Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	2.18%
Duration	4 years 6 months
Current Yield	0.21%
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	1.24%
Current Yield	0.10%
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	1.69%
Current Yield	0.48%
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	1.45%
Duration	4 years
Current Yield	0.28%
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	2.91%
Duration	7 years
Current Yield	0.79%
Auction Rate Securities Purchase Commitment [Member] | Weighted Average [Member] | Municipal Auction Rate Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	2.18%
Duration	4 years 6 months
Current Yield	0.21%
Auction Rate Securities [Member] | Fair Value, Valuation Scenario Two [Member]
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	0.50%
Auction Rate Securities [Member] | Fair Value, Valuation Scenario One [Member]
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	0.25%
Auction Rate Securities Purchase Commitment [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	14,250,000
Valuation Adjustment	797,000
Fair Value	13,453,000
Auction Rate Securities Purchase Commitment [Member] | Auction Rate Preferred Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	10,116,000
Valuation Adjustment	449,000
Fair Value	9,667,000
Valuation Technique	Discounted Cash Flow
Auction Rate Securities Purchase Commitment [Member] | Student Loan Auction Rate Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	18,000
Valuation Adjustment	2,000
Fair Value	16,000
Valuation Technique	Discounted Cash Flow
Auction Rate Securities Purchase Commitment [Member] | Municipal Auction Rate Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	4,116,000
Valuation Adjustment	346,000
Fair Value	3,770,000
Valuation Technique	Discounted Cash Flow