Fair Value Measurements - Quantitative Information about Level 3 Fair Value Measurements (Details)	12 Months Ended
	Dec. 31, 2016 USD ($)
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	$ 119,137,000
Valuation Adjustment	(2,995,000)
Fair Value	$ 116,142,000
Auction Rate Preferred Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
U S Treasury Rate	1.69%
Municipal Auction Rate Securities 01
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Spread To U S Treasury Rate	175.00%
U S Treasury Rate	1.81%
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	2.25%
Auction Rate Securities Owned
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	$ 88,125,000	[1]
Valuation Adjustment	(3,199,000)	[1]
Fair Value	84,926,000	[1]
Auction Rate Securities Owned | Auction Rate Preferred Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	87,800,000	[1]
Valuation Adjustment	(3,171,000)	[1]
Fair Value	$ 84,629,000	[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	$ 25,000	[1]
Valuation Adjustment	(1,000)	[1]
Fair Value	$ 24,000	[1]
Valuation Technique	Discounted Cash Flow	[1]
Discount Rate	3.16%	[1],[2]
Duration	4 years 6 months	[1]
Fair Value Unobservable Input Current Yield	2.05%	[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	(27,000)	[1]
Fair Value	$ 273,000	[1]
Valuation Technique	Discounted Cash Flow	[1]
Discount Rate	3.45%	[1],[4]
Duration	7 years	[1]
Fair Value Unobservable Input Current Yield	1.98%	[1],[3]
Auction Rate Securities Purchase Commitment | Municipal Auction Rate Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	3.16%	[2],[5]
Duration	4 years 6 months	[5]
Fair Value Unobservable Input Current Yield	2.05%	[3],[5]
Auction Rate Securities Purchase Commitment | Student Loan Auction Rate Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	3.45%	[4],[5]
Duration	7 years	[5]
Fair Value Unobservable Input Current Yield	1.98%	[3],[5]
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	1.86%	[1],[2]
Fair Value Unobservable Input Current Yield	1.18%	[1],[2]
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	2.53%	[1],[2]
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	1.27%	[3],[5]
Weighted Average | Auction Rate Securities Owned | Auction Rate Preferred Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	2.18%	[1],[6]
Duration	4 years	[1]
Fair Value Unobservable Input Current Yield	1.23%	[1]
Weighted Average | Auction Rate Securities Owned | Municipal Auction Rate Securities 01
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	3.16%	[1],[2]
Duration	4 years 6 months	[1]
Fair Value Unobservable Input Current Yield	2.05%	[1],[3]
Weighted Average | Auction Rate Securities Owned | Student Loan Auction Rate Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	3.45%	[1],[4]
Duration	7 years	[1]
Fair Value Unobservable Input Current Yield	1.98%	[1],[3]
Weighted Average | Auction Rate Securities Purchase Commitment | Municipal Auction Rate Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	3.16%	[2],[5]
Duration	4 years 6 months	[5]
Fair Value Unobservable Input Current Yield	2.05%	[3],[5]
Weighted Average | Auction Rate Securities Purchase Commitment | Student Loan Auction Rate Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	3.45%	[4],[5]
Duration	7 years	[5]
Fair Value Unobservable Input Current Yield	1.98%	[3],[5]
Auction Rate Securities Purchase Commitment
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	$ 31,012,000	[1]
Valuation Adjustment	(204,000)	[1]
Fair Value	31,216,000	[1]
Auction Rate Securities Purchase Commitment | Municipal Auction Rate Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	2,000	[5]
Valuation Adjustment	0	[5]
Fair Value	$ 2,000	[5]
Valuation Technique	Discounted Cash Flow	[5]
Auction Rate Securities Purchase Commitment | Student Loan Auction Rate Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	$ 27,000	[5]
Valuation Adjustment	(2,000)	[5]
Fair Value	$ 25,000	[5]
Valuation Technique	Discounted Cash Flow	[5]
Other Assets | Auction Rate Securities Purchase Commitment | Auction Rate Preferred Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Duration	4 years	[5]
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	1.86%	[1],[2]
Fair Value Unobservable Input Current Yield	1.18%	[1],[2]
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	2.53%	[1],[2]
Fair Value Unobservable Input Current Yield	1.27%	[3],[5]
Other Assets | Weighted Average | Auction Rate Securities Purchase Commitment | Auction Rate Preferred Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	2.18%	[5],[6]
Duration	4 years	[5]
Fair Value Unobservable Input Current Yield	1.23%	[3],[5]
Other Assets | Auction Rate Securities Purchase Commitment | Auction Rate Preferred Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	$ 6,654,000	[5]
Valuation Adjustment	(849,000)	[5]
Fair Value	$ 7,503,000	[5]
Valuation Technique	Discounted Cash Flow	[5]
Accounts Payable and Other Liabilities [Member] | Auction Rate Securities Purchase Commitment | Auction Rate Preferred Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Duration	4 years	[5]
Accounts Payable and Other Liabilities [Member] | Minimum [Member] | Auction Rate Securities Purchase Commitment | Auction Rate Preferred Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	1.86%	[1],[2]
Fair Value Unobservable Input Current Yield	1.18%	[1],[2]
Accounts Payable and Other Liabilities [Member] | Maximum [Member] | Auction Rate Securities Purchase Commitment | Auction Rate Preferred Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	2.53%	[1],[2]
Fair Value Unobservable Input Current Yield	1.27%	[3],[5]
Accounts Payable and Other Liabilities [Member] | Weighted Average | Auction Rate Securities Purchase Commitment | Auction Rate Preferred Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	2.18%	[5],[6]
Duration	4 years	[5]
Fair Value Unobservable Input Current Yield	1.23%	[3],[5]
Accounts Payable and Other Liabilities [Member] | Auction Rate Securities Purchase Commitment | Auction Rate Preferred Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	$ 24,329,000	[5]
Valuation Adjustment	(643,000)	[5]
Fair Value	$ 23,686,000	[5]
Valuation Technique	Discounted Cash Flow	[5]

[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, 2016. The valuation adjustment amount is included as a reduction to securities owned on the consolidated balance sheet as of December 31, 2016.

[2] Derived by applying a multiple to the spread of 175% to the U.S. Treasury rate of 1.81%.

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

[4] Derived by applying the sum of the spread of 1.20% to the U.S. Treasury rate of 2.25%.

[5] 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 amounts, unrealized gains and losses, are included in other assets and accounts payable and other liabilities, respectively, on the consolidated balance sheet as of December 31, 2016.

[6] Derived by applying a multiple to the spread between 110% to 150% to the U.S. Treasury rate of 1.69%.