Fair Value Measurements - Additional Information (Narrative) (Details) - USD ($)	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities owned, at fair value	$ 837,584,000	$ 926,597,000
ARS Valuation Adjustment For Level Two and Three Assets	$ 3,794,000
Loan position held in secondary loan trading portfolio		0
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	$ 28,215,000	34,067,000
Auction rate securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities owned, at fair value	37,952,000	111,853,000
Auction Rate Securities Owned
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
ARS Valuation Adjustment For Level Two and Three Assets	2,698,000
Auction rate securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total amount of ARS the firm purchased and hold	40,700,000
Amount of ARS committed to purchase from clients	7,300,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers, Net	$ (37,675,000)	(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	2.60%
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]
Fair Value Input, Discount Rate	2.86%
Fair Value Unobservable Input Current Yield	2.69%
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	4.05%
Auction Rate Preferred Securities \| Maximum [Member] \| Auction Rate Securities Owned
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value Input, Discount Rate	3.89%
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	2.48%
Student Loan Auction Rate Securities \| Auction Rate Securities Owned
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value Input, Discount Rate	3.68%
Fair Value Unobservable Input Current Yield	3.64%
Auction Rate Securities Purchase Commitment
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Valuation Adjustment For Level Two Assets	$ 1,096,000
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	$ 22,000,000
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	$ 76,000,000
Other Assets \| Auction Rate Preferred Securities \| Minimum [Member] \| Auction Rate Securities Purchase Commitment
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value Input, Discount Rate	2.86%
Fair Value Unobservable Input Current Yield	2.69%
Other Assets \| Auction Rate Preferred Securities \| Maximum [Member] \| Auction Rate Securities Purchase Commitment
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value Input, Discount Rate	3.89%
Fair Value Unobservable Input Current Yield	4.05%
Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities owned, at fair value	$ 129,643,000
Level 2 \| Auction rate securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities owned, at fair value	$ 16,253,000	$ 24,455,000

ARS Valuation Adjustment For Level Two and Three Assets

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The total amount of ARS the firm purchased and hold.

+ References

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The amount of ARS the firm committed to purchase from clients.

+ References

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Fair Value Input, Discount Rate

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Fair value unobservable input current yield.

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- Definition

Investment redemption notice period maximum.

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Investment redemption notice period minimum.

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Investments lock in period.

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- Definition

Loan position held in secondary loan trading portfolio fair value.

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- Definition

Maximum settlement period in case of delayed delivery of underlying instrument.

+ References

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Securities committed to purchase period.

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- Definition

Sensitivity analysis of fair value of interests continued to be held by transferor servicing assets or liabilities change in discount rate.

+ References

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- Definition

Spread represents ARS maximum formula which is used to discount for the lack of liquidity as a result of ARS failed auction.

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Specific period of US Treasury rate as of reporting period.

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- Definition

Valuation Adjustment For Level Two Assets

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Amount of transfer of financial instrument classified as an asset into (out of) level 3 of the fair value hierarchy.

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- Definition

Fair value of financial instruments held by a broker-dealer for their own account (proprietary securities) for trading or investment purposes determined by management based upon quoted prices of a financial instrument with similar characteristics or on a valuation technique or model.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 940
-SubTopic 320
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=35711157&loc=d3e42546-110969

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The hypothetical financial impact of a specified adverse percentage change in discount rate on the fair value of transferor's interests in transferred financial assets (including any servicing assets or servicing liabilities) as of the balance sheet date.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=109247956&loc=d3e107314-111719

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