Business Segments - Revenue and Pre-tax Earnings by Segment (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Segment Reporting Information [Line Items]
Revenues	$ 1,454,412	$ 1,599,110	$ 1,316,232
Pre-tax earnings	396,428	379,962	85,187
Total assets	6,261,287	6,040,535	6,821,375
Market Making
Segment Reporting Information [Line Items]
Revenues	775,173	884,858	901,152
Pre-tax earnings	93,732	124,028	146,713
Total assets	5,400,530	4,855,482	4,401,021
Global Execution Services
Segment Reporting Information [Line Items]
Revenues	283,756	667,723	345,710
Pre-tax earnings	12,008	368,957	11,056
Total assets	703,426	727,029	786,734
Corporate and Other
Segment Reporting Information [Line Items]
Revenues	395,483	46,529	69,369
Pre-tax earnings	290,688	(113,023)	(72,582)
Total assets	157,331	458,024	1,633,620
U.S.
Segment Reporting Information [Line Items]
Revenues	1,351,242	1,449,370	1,127,088
International
Segment Reporting Information [Line Items]
Revenues	$ 103,170	$ 149,740	$ 189,144

Sum of the carrying amounts as of the balance sheet date of all assets that are recognized. Assets are probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(12))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
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-Section S99
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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 20
-Section 25
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-URI http://asc.fasb.org/extlink&oid=68072869&loc=d3e41242-110953

Reference 2: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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-SubTopic 10
-Section S99
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Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

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