Fair Value Measurements - Additional Information (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2018	Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Level Three Principal Amount	$ 48,033	$ 48,033
Securities owned, at fair value	895,251	$ 895,251	$ 926,597
Forward or delayed delivery of the underlying instrument with settlement		180 days
Valuation Adjustment For Level Three Asset and Liability	4,196	$ 4,196
Fair Value Of Level Three Assets and Liabilities	43,837	43,837
Equity Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities owned, at fair value	34,613	34,613	34,067
Auction rate securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Level Three Principal Amount	40,800	40,800
Securities owned, at fair value	37,683	37,683	$ 111,853
Valuation adjustment (unrealized loss) for ARS	(3,117)	(3,117)
Total amount of ARS the firm purchased and hold		40,800
Fair Value Of Level Three Assets	$ 37,683	$ 37,683
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%	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%	50.00%
Auction Rate Preferred Securities 01 [Member] | Auction rate securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Level Three Principal Amount	$ 18,950	$ 18,950
Valuation adjustment (unrealized loss) for ARS	(2,697)	(2,697)
Fair Value Of Level Three Assets	16,253	16,253
Total Losses from Tender Offer	8,100
Realized losses on auction rate securities	4,600
Unrealized losses on auction rate securities	3,500
Municipal Auction Rate Securities [Member] | Auction rate securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Level Three Principal Amount	75	75
Valuation adjustment (unrealized loss) for ARS	0	0
Fair Value Of Level Three Assets	$ 75	$ 75
Auction Rate Preferred Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
U S Treasury Rate	2.85%	2.85%
Auction Rate Preferred Securities | Auction rate securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Level Three Principal Amount	$ 21,500	$ 21,500
Fair Value Unobservable Inputs Duration		2 years 6 months
Valuation adjustment (unrealized loss) for ARS	(406)	$ (406)
Fair Value Of Level Three Assets	$ 21,094	$ 21,094
Auction Rate Preferred Securities | Weighted Average [Member] | Auction rate securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value Unobservable Inputs Duration		2 years 6 months
Fair Value Unobservable Input Current Yield	3.45%	3.45%
Auction Rate Preferred Securities | Minimum [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Spread To U S Treasury Rate	110.00%	110.00%
Auction Rate Preferred Securities | Minimum [Member] | Auction rate securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value Unobservable Input Current Yield	2.38%	2.38%
Auction Rate Preferred Securities | Maximum [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Spread To U S Treasury Rate	150.00%	150.00%
Auction Rate Preferred Securities | Maximum [Member] | Auction rate securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value Unobservable Input Current Yield	3.46%	3.46%
Student Loan Auction Rate Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Spread To U S Treasury Rate	1.20%	1.20%
U S Treasury Rate	2.97%	2.97%
Student Loan Auction Rate Securities | Auction rate securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Level Three Principal Amount	$ 275	$ 275
Fair Value Unobservable Inputs Duration		5 years 6 months
Valuation adjustment (unrealized loss) for ARS	$ (14)	$ (14)
Fair Value Unobservable Input Current Yield	3.09%	3.09%
Fair Value Of Level Three Assets	$ 261	$ 261
Student Loan Auction Rate Securities | Weighted Average [Member] | Auction rate securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value Unobservable Inputs Duration		5 years 6 months
Fair Value Unobservable Input Current Yield	3.09%	3.09%
Auction Rate Securities Purchase Commitment
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Level Three Principal Amount	$ 7,233	$ 7,233
Auction Rate Securities Committed To Purchase From Client		7,200
Valuation Adjustment For Level Three Liabilities	1,079	1,079
Fair Value Of Level Three Liabilities	6,154	6,154
Auction Rate Securities Purchase Commitment | Auction Rate Preferred Securities 01 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Level Three Principal Amount	7,184	7,184
Valuation Adjustment For Level Three Liabilities	1,078	1,078
Fair Value Of Level Three Liabilities	6,106	6,106
Auction Rate Securities Purchase Commitment | Auction Rate Preferred Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Level Three Principal Amount	49	$ 49
Fair Value Unobservable Inputs Duration		2 years 6 months
Valuation Adjustment For Level Three Liabilities	1	$ 1
Fair Value Of Level Three Liabilities	$ 48	$ 48
Auction Rate Securities Purchase Commitment | Auction Rate Preferred Securities | Weighted Average [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value Unobservable Inputs Duration		2 years 6 months
Fair Value Unobservable Input Current Yield	3.45%	3.45%
Auction Rate Securities Purchase Commitment | Auction Rate Preferred Securities | Minimum [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value Unobservable Input Current Yield	2.38%	2.38%
Auction Rate Securities Purchase Commitment | Auction Rate Preferred Securities | Maximum [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value Unobservable Input Current Yield	3.46%	3.46%