Derivative Instruments - Fair Value of Derivative Instruments (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Equities futures \| Not designated as hedging instruments \| Receivables from broker dealers and clearing organizations
Fair value of derivative instruments on a gross basis
Derivatives Assets, Fair Value	$ 183	$ (505)
Derivatives Assets, Notional	4,836,630	1,985,770
Equities futures \| Not designated as hedging instruments \| Payables to broker dealers and clearing organizations
Fair value of derivative instruments on a gross basis
Derivatives Liabilities, Fair Value	208	(575)
Derivatives Liabilities, Notional	139,033	142,658
Commodity futures \| Not designated as hedging instruments \| Receivables from broker dealers and clearing organizations
Fair value of derivative instruments on a gross basis
Derivatives Assets, Fair Value	39,965	971
Derivatives Assets, Notional	20,971,297	21,231,001
Commodity futures \| Not designated as hedging instruments \| Payables to broker dealers and clearing organizations
Fair value of derivative instruments on a gross basis
Derivatives Liabilities, Fair Value	645	(1,602)
Derivatives Liabilities, Notional	118,427	130,042
Currency futures \| Not designated as hedging instruments \| Receivables from broker dealers and clearing organizations
Fair value of derivative instruments on a gross basis
Derivatives Assets, Fair Value	32,042	26,548
Derivatives Assets, Notional	2,075,544	3,994,412
Currency futures \| Not designated as hedging instruments \| Payables to broker dealers and clearing organizations
Fair value of derivative instruments on a gross basis
Derivatives Liabilities, Fair Value	(3,279)	(13,947)
Derivatives Liabilities, Notional	2,531,734	7,756,958
Fixed Income futures \| Not designated as hedging instruments \| Receivables from broker dealers and clearing organizations
Fair value of derivative instruments on a gross basis
Derivatives Assets, Fair Value	23	73
Derivatives Assets, Notional	10,096	44,395
Fixed Income futures \| Not designated as hedging instruments \| Payables to broker dealers and clearing organizations
Fair value of derivative instruments on a gross basis
Derivatives Liabilities, Fair Value	(169)	(1)
Derivatives Liabilities, Notional	13,016	2,584
Currency forwards
Fair value of derivative instruments on a gross basis
Derivatives Assets, Fair Value	702,889	2,045,487
Derivatives Liabilities, Fair Value	725,422	2,032,017
Currency forwards \| Not designated as hedging instruments \| Financial instruments owned
Fair value of derivative instruments on a gross basis
Derivatives Assets, Fair Value	702,889	2,045,487
Derivatives Assets, Notional	100,689,302	124,000,221
Currency forwards \| Not designated as hedging instruments \| Financial instruments sold, not yet purchased
Fair value of derivative instruments on a gross basis
Derivatives Liabilities, Fair Value	725,058	2,032,017
Derivatives Liabilities, Notional	100,700,387	123,993,234
Currency forwards \| Derivative instruments designed as hedging instruments: \| Financial instruments sold, not yet purchased
Fair value of derivative instruments on a gross basis
Derivatives Liabilities, Fair Value	(364)	(514)
Derivatives Liabilities, Notional	15,964	16,115
Options
Fair value of derivative instruments on a gross basis
Derivatives Assets, Fair Value	9,210	7,045
Derivatives Liabilities, Fair Value	15,548	5,839
Options \| Not designated as hedging instruments \| Financial instruments owned
Fair value of derivative instruments on a gross basis
Derivatives Assets, Fair Value	9,210	7,045
Derivatives Assets, Notional	793,423	682,369
Options \| Not designated as hedging instruments \| Financial instruments sold, not yet purchased
Fair value of derivative instruments on a gross basis
Derivatives Liabilities, Fair Value	15,548	5,839
Derivatives Liabilities, Notional	$ 769,273	$ 681,147

Nominal or face amount used to calculate payments on the derivative asset.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1A
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=64837297&loc=SL5579245-113959

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Notional Amount
-URI http://asc.fasb.org/extlink&oid=6519104

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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