Fair Value Measurements - Additional Information (Narrative) (Details) - USD ($)	6 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Level Three Principal Amount	$ 104,462,000
Securities owned, at fair value	$ 1,020,404,000	$ 926,597,000
Forward or delayed delivery of the underlying instrument with settlement	180 days
Valuation Adjustment For Level Three Asset and Liability	$ 2,299,000
Fair Value Of Level Three Assets and Liabilities	102,163,000
Equity Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities owned, at fair value	33,112,000	34,067,000
Auction rate securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Level Three Principal Amount	92,300,000
Auction Rate Securities Committed To Purchase related to settlements with regulators	5,000,000
Securities owned, at fair value	90,223,000	$ 111,853,000
Valuation adjustment (unrealized loss) for ARS	(2,077,000)
Total amount of ARS the firm purchased and hold	92,300,000
Fair Value Of Level Three Assets	$ 90,223,000
Maximum [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities Committed To Purchase Period	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 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments Lock In Period	10 years
Fair Value, Valuation Scenario One [Member] | Auction rate securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Basis point	25.00%
Fair Value, Valuation Scenario Two [Member] | Auction rate securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Basis point	50.00%
Municipal Auction Rate Securities [Member] | Auction rate securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Level Three Principal Amount	$ 25,000
Valuation adjustment (unrealized loss) for ARS	0
Fair Value Of Level Three Assets	$ 25,000
Fair Value Measurements, Valuation Techniques	Par
Auction Rate Preferred Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
U S Treasury Rate	2.58%
Auction Rate Preferred Securities | Auction rate securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Level Three Principal Amount	$ 73,275,000
Fair Value Unobservable Inputs Duration	2 years 6 months
Valuation adjustment (unrealized loss) for ARS	$ (941,000)
Fair Value Of Level Three Assets	$ 72,334,000
Fair Value Measurements, Valuation Techniques	Discounted Cash Flow
Auction Rate Preferred Securities | Weighted Average [Member] | Auction rate securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Permanent and construction loans , discount rate	3.22%
Fair Value Unobservable Inputs Duration	2 years 6 months
Fair Value Unobservable Input Current Yield	2.68%
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
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Permanent and construction loans , discount rate	2.83%
Fair Value Unobservable Input Current Yield	2.36%
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
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Permanent and construction loans , discount rate	3.86%
Fair Value Unobservable Input Current Yield	3.21%
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	2.76%
Student Loan Auction Rate Securities | Auction rate securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Level Three Principal Amount	$ 275,000
Permanent and construction loans , discount rate	3.96%
Fair Value Unobservable Inputs Duration	5 years 6 months
Valuation adjustment (unrealized loss) for ARS	$ (13,000)
Fair Value Unobservable Input Current Yield	2.95%
Fair Value Of Level Three Assets	$ 262,000
Fair Value Measurements, Valuation Techniques	Discounted Cash Flow
Student Loan Auction Rate Securities | Weighted Average [Member] | Auction rate securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Permanent and construction loans , discount rate	3.96%
Fair Value Unobservable Inputs Duration	5 years 6 months
Fair Value Unobservable Input Current Yield	2.95%
ARS purchase commitments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Level Three Principal Amount	$ 12,162,000
Amount of ARS committed to purchase from clients	7,200,000
Valuation Adjustment For Level Three Liabilities	222,000
Fair Value Of Level Three Liabilities	11,940,000
ARS purchase commitments | Municipal Auction Rate Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Level Three Principal Amount	$ 2,000
Fair Value Measurements, Valuation Techniques	Par
Valuation Adjustment For Level Three Liabilities	$ 0
Fair Value Of Level Three Liabilities	2,000
ARS purchase commitments | Auction Rate Preferred Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Level Three Principal Amount	$ 10,620,000
Fair Value Unobservable Inputs Duration	2 years 6 months
Fair Value Measurements, Valuation Techniques	Discounted Cash Flow
Valuation Adjustment For Level Three Liabilities	$ 130,000
Fair Value Of Level Three Liabilities	$ 10,490,000
ARS purchase commitments | Auction Rate Preferred Securities | Weighted Average [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Permanent and construction loans , discount rate	3.22%
Fair Value Unobservable Inputs Duration	2 years 6 months
Fair Value Unobservable Input Current Yield	2.68%
ARS purchase commitments | Auction Rate Preferred Securities | Minimum [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Permanent and construction loans , discount rate	2.83%
Fair Value Unobservable Input Current Yield	2.36%
ARS purchase commitments | Auction Rate Preferred Securities | Maximum [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Permanent and construction loans , discount rate	3.86%
Fair Value Unobservable Input Current Yield	3.21%
ARS purchase commitments | Student Loan Auction Rate Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Level Three Principal Amount	$ 25,000
Permanent and construction loans , discount rate	3.96%
Fair Value Unobservable Inputs Duration	5 years 6 months
Fair Value Unobservable Input Current Yield	2.95%
Fair Value Measurements, Valuation Techniques	Discounted Cash Flow
Valuation Adjustment For Level Three Liabilities	$ 1,000
Fair Value Of Level Three Liabilities	$ 24,000
ARS purchase commitments | Student Loan Auction Rate Securities | Weighted Average [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Permanent and construction loans , discount rate	3.96%
Fair Value Unobservable Inputs Duration	5 years 6 months
Fair Value Unobservable Input Current Yield	2.95%