Fair Value Measurements - Quantitative Information about Level 3 Fair Value Measurements (Details)	12 Months Ended
	Dec. 31, 2017 USD ($)
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	$ 124,867,000
Valuation Adjustment	2,030,000
Fair Value	$ 122,837,000
Auction Rate Preferred Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
U S Treasury Rate	2.09%
Auction Rate Preferred Securities 02 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Tender Offer, percentage of Par	95.00%
Municipal Auction Rate Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Spread To U S Treasury Rate	175.00%
U S Treasury Rate	2.15%
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	2.33%
Auction Rate Securities Owned
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	$ 113,875,000	[1]
Valuation Adjustment	2,022,000	[1]
Fair Value	111,853,000	[1]
Auction Rate Securities Owned | Auction Rate Preferred Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	88,025,000	[1]
Valuation Adjustment	1,318,000	[1]
Fair Value	$ 86,707,000	[1]
Valuation Technique	Discounted Cash Flow	[1]
Duration	4 years	[1]
Auction Rate Securities Owned | Auction Rate Preferred Securities 01 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	$ 11,725,000	[1]
Valuation Adjustment	0	[1]
Fair Value	$ 11,725,000	[1]
Valuation Technique	Par (7)	[1]
Auction Rate Securities Owned | Auction Rate Preferred Securities 02 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	$ 13,400,000	[1]
Valuation Adjustment	670,000	[1]
Fair Value	$ 12,730,000	[1]
Valuation Technique	Tender Offer (8)	[1]
Auction Rate Securities Owned | Municipal Auction Rate Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	$ 425,000	[1]
Valuation Adjustment	23,000	[1]
Fair Value	$ 402,000	[1]
Valuation Technique	Discounted Cash Flow	[1]
Discount Rate	3.76%	[1],[2]
Duration	4 years 6 months	[1]
Fair Value Unobservable Input Current Yield	2.41%	[1],[3]
Auction Rate Securities Owned | Student Loan Auction Rate Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	$ 300,000	[1]
Valuation Adjustment	11,000	[1]
Fair Value	$ 289,000	[1]
Valuation Technique	Discounted Cash Flow	[1]
Discount Rate	3.53%	[1],[2]
Duration	7 years	[1]
Fair Value Unobservable Input Current Yield	2.93%	[1],[3]
Auction Rate Securities Purchase Commitment | Auction Rate Preferred Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Duration	4 years	[4]
Minimum [Member] | Auction Rate Preferred Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Spread To U S Treasury Rate	110.00%
Minimum [Member] | Auction Rate Securities Owned | Auction Rate Preferred Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	2.30%	[1],[5]
Fair Value Unobservable Input Current Yield	2.37%	[1],[5]
Maximum [Member] | Auction Rate Preferred Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Spread To U S Treasury Rate	150.00%
Maximum [Member] | Auction Rate Securities Owned | Auction Rate Preferred Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	3.13%	[1],[5]
Maximum [Member] | Auction Rate Securities Purchase Commitment | Auction Rate Preferred Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Unobservable Input Current Yield	2.47%	[3],[4]
Weighted Average | Auction Rate Securities Owned | Auction Rate Preferred Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	2.74%	[1],[6]
Duration	4 years	[1]
Fair Value Unobservable Input Current Yield	2.41%	[1]
Weighted Average | Auction Rate Securities Owned | Student Loan Auction Rate Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	3.53%	[1],[2]
Duration	7 years	[1]
Fair Value Unobservable Input Current Yield	2.93%	[1],[3]
Weighted Average | Municipal Auction Rate Securities | Auction Rate Preferred Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	3.76%	[1],[6]
Duration	4 years 6 months	[1]
Fair Value Unobservable Input Current Yield	2.41%	[1]
Weighted Average | Auction Rate Securities Purchase Commitment | Auction Rate Preferred Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	2.74%	[4],[6]
Duration	4 years	[4]
Fair Value Unobservable Input Current Yield	2.41%	[3],[4]
Auction Rate Securities Purchase Commitment
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	$ 10,992,000	[1]
Valuation Adjustment	8,000	[1]
Fair Value	10,984,000	[1]
Auction Rate Securities Purchase Commitment | Auction Rate Preferred Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	10,992,000	[4]
Valuation Adjustment	8,000	[4]
Fair Value	$ 10,984,000	[4]
Valuation Technique	Discounted Cash Flow	[4]
Other Assets | Minimum [Member] | Auction Rate Securities Purchase Commitment | Auction Rate Preferred Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	2.30%	[1],[5]
Fair Value Unobservable Input Current Yield	2.37%	[1],[5]
Other Assets | Maximum [Member] | Auction Rate Securities Purchase Commitment | Auction Rate Preferred Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	3.13%	[1],[5]
Fair Value Unobservable Input Current Yield	2.47%	[3],[4]

[1] Principal amount represents the par value of the ARS and is included in securities owned on the consolidated balance sheet as of December 31, 2017. The valuation adjustment amount is included as a reduction to securities owned on the consolidated balance sheet as of December 31, 2017.

[2] Derived by applying the sum of the spread of 1.20% to the U.S. Treasury rate of 2.33%.

[3] Based on current yields for ARS positions owned.

[4] 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 as of December 31, 2017

[5] (4)Derived by applying a multiple to the spread of 175% to the U.S. Treasury rate of 2.15%.

[6] Derived by applying a multiple to the spread between 110% to 150% to the U.S. Treasury rate of 2.09%.