Fair Value Measurements - Additional Information (Narrative) (Details) - USD ($)	3 Months Ended
	Mar. 31, 2017	Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities owned, at fair value	$ 1,002,705,000	$ 707,108,000
Valuation adjustment (unrealized loss) for ARS	(2,038,000)
Fair value of the reverse repurchase agreements	3,896,000	24,006,000
Fair value of the repurchase agreements	$ 0
Forward or delayed delivery of the underlying instrument with settlement	180 days
Auction rate securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities owned, at fair value	$ 89,743,000	$ 84,926,000
Auction Rate Securities Owned
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Valuation adjustment (unrealized loss) for ARS	$ (2,557,000)
Senior Secured Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Interest rate	8.75%
Auction rate securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets and liabilities transferred from level one and level two	$ 0
Total amount of ARS the firm purchased and hold	92,300,000
Amount of ARS committed to purchase from clients	$ 26,400,000
Auction rate securities | Maximum [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Commitment to purchase ARS period maximum	2020
Hedge Funds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investment Redemption Notice Period Minimum	30 days
Investment Redemption Notice Period Maximum	120 days
Private Equity Funds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments Lock In Period	10 years
Auction Rate Preferred Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
U S Treasury Rate	1.71%
Auction Rate Preferred Securities | Auction Rate Securities Owned
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Valuation adjustment (unrealized loss) for ARS	$ (2,530,000)
Auction Rate Preferred Securities | Minimum [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Spread To U S Treasury Rate	110.00%
Auction Rate Preferred Securities | Minimum [Member] | Auction Rate Securities Owned
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Permanent and construction loans , discount rate	1.88%
Fair Value Unobservable Input Current Yield	1.40%
Auction Rate Preferred Securities | Maximum [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Spread To U S Treasury Rate	150.00%
Auction Rate Preferred Securities | Maximum [Member] | Auction Rate Securities Purchase Commitment
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value Unobservable Input Current Yield	1.56%
Auction Rate Preferred Securities | Maximum [Member] | Auction Rate Securities Owned
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Permanent and construction loans , discount rate	2.56%
Student Loan Auction Rate Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Spread To U S Treasury Rate	1.20%
U S Treasury Rate	2.21%
Student Loan Auction Rate Securities | Auction Rate Securities Owned
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Permanent and construction loans , discount rate	3.41%
Valuation adjustment (unrealized loss) for ARS	$ (26,000)
Fair Value Unobservable Input Current Yield	1.98%
Auction Rate Securities Purchase Commitment
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Valuation adjustment (unrealized loss) for ARS	$ (519,000)
Fair Value, Valuation Scenario One [Member] | Auction rate securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Basis point	25.00%
Decrease in fair value of ARS	$ 1,100,000
Fair Value, Valuation Scenario Two | Auction rate securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Basis point	50.00%
Decrease in fair value of ARS	$ 2,200,000
Other Assets | Auction Rate Preferred Securities | Minimum [Member] | Auction Rate Securities Purchase Commitment
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Permanent and construction loans , discount rate	1.88%
Fair Value Unobservable Input Current Yield	1.40%
Other Assets | Auction Rate Preferred Securities | Maximum [Member] | Auction Rate Securities Purchase Commitment
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Permanent and construction loans , discount rate	2.56%
Fair Value Unobservable Input Current Yield	1.56%
Other Assets | Auction Rate Securities Purchase Commitment | Auction Rate Preferred Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Valuation adjustment (unrealized loss) for ARS	$ (878,000)
Accounts Payable and Other Liabilities [Member] | Auction Rate Preferred Securities | Minimum [Member] | Auction Rate Securities Purchase Commitment
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Permanent and construction loans , discount rate	1.88%
Fair Value Unobservable Input Current Yield	1.40%
Accounts Payable and Other Liabilities [Member] | Auction Rate Preferred Securities | Maximum [Member] | Auction Rate Securities Purchase Commitment
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Permanent and construction loans , discount rate	2.56%
Fair Value Unobservable Input Current Yield	1.56%
Accounts Payable and Other Liabilities [Member] | Auction Rate Securities Purchase Commitment | Auction Rate Preferred Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Valuation adjustment (unrealized loss) for ARS	$ (359,000)
Accounts Payable and Other Liabilities [Member] | Auction Rate Securities Purchase Commitment | Auction Rate Preferred Securities and Student Loan Auction Rate Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Valuation adjustment (unrealized loss) for ARS	$ (359,000)