Fair Value Measurements - Additional Information (Narrative) (Details) - USD ($)	3 Months Ended
Mar. 31, 2018	Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities owned, at fair value	$ 999,627,000	$ 926,597,000
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	$ 31,974,000	34,067,000
Auction rate securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities owned, at fair value	87,350,000	$ 111,853,000
Auction Rate Securities Owned
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Valuation adjustment (unrealized loss) for ARS	(1,175,000)
Auction rate securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Auction Rate Securities Committed To Purchase related to settlements with regulators	5,000,000
Assets and liabilities transferred from level one and level two	0
Total amount of ARS the firm purchased and hold	88,500,000
Amount of ARS committed to purchase from clients	$ 11,000,000
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 [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%
Decrease in fair value of ARS	$ 614,000
Fair Value, Valuation Scenario Two [Member] \| Auction rate securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Basis point	50.00%
Decrease in fair value of ARS	$ 1,200,000
Auction Rate Preferred Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
U S Treasury Rate	2.32%
Auction Rate Preferred Securities \| Auction Rate Securities Owned
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Valuation adjustment (unrealized loss) for ARS	$ (1,158,000)
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	2.56%
Fair Value Unobservable Input Current Yield	2.15%
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 Owned
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Permanent and construction loans , discount rate	3.49%
Fair Value Unobservable Input Current Yield	2.79%
Auction Rate Securities Purchase Commitment \| Auction Rate Preferred Securities \| Minimum [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Permanent and construction loans , discount rate	2.56%
Fair Value Unobservable Input Current Yield	2.15%
Auction Rate Securities Purchase Commitment \| Auction Rate Preferred Securities \| Maximum [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Permanent and construction loans , discount rate	3.49%
Fair Value Unobservable Input Current Yield	2.79%

The total amount of ARS the firm purchased and hold.

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- Definition

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

+ References

No definition available.

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- Definition

Auction Rate Securities Committed To Purchase related to settlements with regulators

+ References

No definition available.

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- Definition

Fair value unobservable input current yield.

+ References

No definition available.

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- Definition

Investment redemption notice period maximum.

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- Definition

Investment redemption notice period minimum.

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- Definition

Investments lock in period.

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No definition available.

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- Definition

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

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- Definition

Securities committed to purchase period.

+ References

No definition available.

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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

No definition available.

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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.

+ References

No definition available.

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- Definition

Specific period of US Treasury rate as of reporting period.

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- Definition

Valuation adjustment for the level 3 assets used for quantitative disclosure.

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- Definition

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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No definition available.

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- Definition

Interest rate used to find the present value of an amount to be paid or received in the future as an input to measure fair value. For example, but not limited to, weighted average cost of capital (WACC), cost of capital, cost of equity and cost of debt.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

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- Definition

Amount of transfers of financial instrument classified as a derivative asset (liability) after deduction of derivative liability (asset) into (out of) level 3 of the fair value hierarchy.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19279-110258

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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://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13433-108611

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-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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- Definition

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://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=82924980&loc=d3e107314-111719

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