Fair Value Measurements - Additional Information (Narrative) (Details) - USD ($)		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities owned, at fair value		$ 926,597,000	$ 707,108,000
Valuation adjustment (unrealized loss) for ARS		$ (2,030,000)
Loan position held in secondary loan trading portfolio			0
Fair value of the reverse repurchase agreements			24,006,000
Forward or delayed delivery of the underlying instrument with settlement		180 days
Equity Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities owned, at fair value		$ 34,067,000	31,174,000
Auction rate securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities owned, at fair value		111,853,000	84,926,000
Auction Rate Securities Owned
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Valuation adjustment (unrealized loss) for ARS	[1]	(2,022,000)
Auction rate securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total amount of ARS the firm purchased and hold		113,900,000
Amount of ARS committed to purchase from clients		11,000,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers, Net		$ (24,455,000)
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		2.09%
Auction Rate Preferred Securities | Auction Rate Securities Owned
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Valuation adjustment (unrealized loss) for ARS	[1]	$ (1,318,000)
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],[2]	2.30%
Fair Value Unobservable Input Current Yield	[1],[2]	2.37%
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	[3],[4]	2.47%
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	[1],[2]	3.13%
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		2.33%
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	[1],[5]	3.53%
Valuation adjustment (unrealized loss) for ARS	[1]	$ (11,000)
Fair Value Unobservable Input Current Yield	[1],[3]	2.93%
Auction Rate Securities Purchase Commitment
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Valuation adjustment (unrealized loss) for ARS	[1]	$ (8,000)
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	[4]	$ (8,000)
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		$ 430,000,000
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		$ 856,000,000
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],[2]	2.30%
Fair Value Unobservable Input Current Yield	[1],[2]	2.37%
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	[1],[2]	3.13%
Fair Value Unobservable Input Current Yield	[3],[4]	2.47%
Fair Value, Inputs, Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities owned, at fair value		$ 161,366,000
Fair value of the reverse repurchase agreements			$ 24,006,000
Fair Value, Inputs, Level 2 [Member] | Auction rate securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities owned, at fair value		24,455,000
Fair Value, Inputs, Level 2 [Member] | Auction rate securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers, Net		$ 24,500,000

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

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