Fair Value Measurements - Securities Owned and Securities Sold, But Not Yet Purchased at Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	$ 1,040,631	$ 843,155
Securities Sold	311,664	92,510
US Treasury and Government [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	750,031	570,607
Securities Sold	259,810	30,615
Corporate Debt and Other Obligations [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	15,475	19,795
Securities Sold	7,070	2,646
Mortgage and Other Asset-Backed Securities [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	7,276	6,689
Securities Sold	28	255
Municipal Obligations [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	74,365	60,833
Securities Sold	50	51
Convertible Bonds [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	54,946	49,813
Securities Sold	9,539	11,369
Corporate Equities [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	41,209	42,751
Securities Sold	35,167	47,574
Money Markets [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	846	1,245
Securities Sold	0	0
Auction Rate Securities [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	96,483	91,422
Securities Sold	$ 0	$ 0

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.

No definition available.

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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=49121117&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

Fair value of financial instruments sold short 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

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