Fair Value Measurements - Quantitative Information about Level 3 Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	$ 110,799
Valuation Adjustment	8,030
Fair Value	102,769
Auction Rate Securities Owned
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	98,550	[1]
Valuation Adjustment	7,128	[1]
Fair Value	91,422	[1]
Auction Rate Securities Owned | Auction Rate Preferred Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	77,025	[1]
Valuation Adjustment	4,339	[1]
Fair Value	72,686	[1]
Valuation Technique	Discounted Cash Flow	[1]
Duration	4 years	[1]
Auction Rate Securities Owned | Municipal Auction Rate Securities 01
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	11,475	[1]
Valuation Adjustment	1,157	[1]
Fair Value	10,318	[1]
Valuation Technique	Discounted Cash Flow	[1]
Discount Rate	2.64%	[1],[2]
Duration	4 years 6 months	[1]
Current Yield	0.23%	[1],[3]
Auction Rate Securities Owned | Municipal Auction Rate Securities 02
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	5,975	[1]
Valuation Adjustment	627	[1]
Fair Value	5,348	[1]
Valuation Technique	Secondary Market Trading Activity	[1]
Observable trades in inactive market for in portfolio securities	89.50%	[1]
Auction Rate Securities Owned | Student Loan Auction Rate Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	450	[1]
Valuation Adjustment	66	[1]
Fair Value	384	[1]
Valuation Technique	Discounted Cash Flow	[1]
Discount Rate	3.17%	[1],[4]
Duration	7 years	[1]
Current Yield	0.79%	[1],[3]
Auction Rate Securities Owned | Other Auction Rate Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	3,625	[1],[5]
Valuation Adjustment	939	[1],[5]
Fair Value	2,686	[1],[5]
Valuation Technique	Secondary Market Trading Activity	[1],[5]
Observable trades in inactive market for in portfolio securities	74.11%	[1],[5]
Auction Rate Securities Purchase Commitment | Auction Rate Preferred Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Duration	4 years	[6]
Auction Rate Securities Purchase Commitment | Municipal Auction Rate Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	2.64%	[2],[6]
Duration	4 years 6 months	[6]
Current Yield	0.23%	[3],[6]
Auction Rate Securities Purchase Commitment | Student Loan Auction Rate Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	3.17%	[4],[6]
Duration	7 years	[6]
Current Yield	0.79%	[3],[6]
Minimum | Auction Rate Securities Owned | Auction Rate Preferred Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	1.50%	[1],[7]
Current Yield	0.012%	[1],[3]
Minimum | Auction Rate Securities Purchase Commitment | Auction Rate Preferred Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	1.50%	[6],[7]
Current Yield	0.12%	[3],[6]
Maximum | Auction Rate Securities Owned | Auction Rate Preferred Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	2.04%	[1],[7]
Current Yield	0.48%	[1],[3]
Maximum | Auction Rate Securities Purchase Commitment | Auction Rate Preferred Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	2.04%	[6],[7]
Current Yield	0.48%	[3],[6]
Weighted Average | Auction Rate Securities Owned | Auction Rate Preferred Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	1.78%	[1],[7]
Duration	4 years	[1]
Current Yield	0.30%	[1]
Weighted Average | Auction Rate Securities Owned | Municipal Auction Rate Securities 01
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	2.64%	[1],[2]
Duration	4 years 6 months	[1]
Current Yield	0.23%	[1],[3]
Weighted Average | Auction Rate Securities Owned | Municipal Auction Rate Securities 02
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Observable trades in inactive market for in portfolio securities	89.50%	[1]
Weighted Average | Auction Rate Securities Owned | Student Loan Auction Rate Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	3.17%	[1],[4]
Duration	7 years	[1]
Current Yield	0.79%	[1],[3]
Weighted Average | Auction Rate Securities Owned | Other Auction Rate Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Observable trades in inactive market for in portfolio securities	74.11%	[1],[5]
Weighted Average | Auction Rate Securities Purchase Commitment | Auction Rate Preferred Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	1.78%	[6],[7]
Duration	4 years	[6]
Current Yield	0.30%	[3],[6]
Weighted Average | Auction Rate Securities Purchase Commitment | Municipal Auction Rate Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	2.64%	[2],[6]
Duration	4 years 6 months	[6]
Current Yield	0.23%	[3],[6]
Weighted Average | Auction Rate Securities Purchase Commitment | Student Loan Auction Rate Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	3.17%	[4],[6]
Duration	7 years	[6]
Current Yield	0.79%	[3],[6]
Auction Rate Securities Purchase Commitment
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	12,249	[1]
Valuation Adjustment	902	[1]
Fair Value	11,347	[1]
Auction Rate Securities Purchase Commitment | Auction Rate Preferred Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	7,626	[6]
Valuation Adjustment	410	[6]
Fair Value	7,216	[6]
Valuation Technique	Discounted Cash Flow	[6]
Auction Rate Securities Purchase Commitment | Municipal Auction Rate Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	4,065	[6]
Valuation Adjustment	410	[6]
Fair Value	3,655	[6]
Valuation Technique	Discounted Cash Flow	[6]
Auction Rate Securities Purchase Commitment | Student Loan Auction Rate Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	558	[6]
Valuation Adjustment	82	[6]
Fair Value	$ 476	[6]
Valuation Technique	Discounted Cash Flow	[6]

[1] Principal amount represents the par value of the ARS and is included in securities owned in the consolidated balance sheet at December 31, 2014. The valuation adjustment amount is included as a reduction to securities owned in the consolidated balance sheet at December 31, 2014.

[2] Derived by applying a multiple to the spread of 175% to the U.S. Treasury rate of 1.51%.

[3] Based on current auctions in comparable securities that have not failed.

[4] Derived by applying the sum of the spread of 1.20% to the U.S. Treasury rate of 1.97%.

[5] Represents ARS issued by credit default obligation structure that the Company has purchased and is committed to purchase as a result of a legal settlement.

[6] Principal amount represents the present value of the ARS par value that the Company is committed to purchase at a future date. This principal amount is presented as an off-balance sheet item. The valuation adjustment amount is included in accounts payable and other liabilities on the consolidated balance sheet at December 31, 2014.(7)

[7] Derived by applying a multiple to the spread between 110% to 150% to the U.S. Treasury rate of 1.36%.