Derivative Instruments - Fair Value of Derivative Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Equities futures \| Receivables from broker dealers and clearing organizations
Fair value of derivative instruments on a gross basis
Derivatives Assets, Fair Value	$ (505)	$ 2,403
Derivatives Assets, Notional	1,985,770	1,461,286
Equities futures \| Payables to broker dealers and clearing organizations
Fair value of derivative instruments on a gross basis
Derivatives Liabilities, Fair Value	(575)	(43)
Derivatives Liabilities, Notional	142,658	62,417
Commodity futures \| Receivables from broker dealers and clearing organizations
Fair value of derivative instruments on a gross basis
Derivatives Assets, Fair Value	971	13,964
Derivatives Assets, Notional	21,231,001	3,918,778
Commodity futures \| Payables to broker dealers and clearing organizations
Fair value of derivative instruments on a gross basis
Derivatives Liabilities, Fair Value	(1,602)	2,842
Derivatives Liabilities, Notional	130,042	22,616,170
Currency futures \| Receivables from broker dealers and clearing organizations
Fair value of derivative instruments on a gross basis
Derivatives Assets, Fair Value	26,548	1,591
Derivatives Assets, Notional	3,994,412	3,264,093
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	(13,947)	(6,282)
Derivatives Liabilities, Notional	7,756,958	1,137,908
Fixed Income futures \| Not designated as hedging instruments
Fair value of derivative instruments on a gross basis
Derivatives Liabilities, Fair Value	(1)
Derivatives Liabilities, Notional	2,584
Currency forwards
Fair value of derivative instruments on a gross basis
Derivatives Assets, Fair Value	2,045,487	1,147,261
Derivatives Liabilities, Fair Value	2,032,017	1,009,038
Currency forwards \| Financial instruments owned
Fair value of derivative instruments on a gross basis
Derivatives Assets, Fair Value	2,045,487	1,147,261
Derivatives Assets, Notional	124,000,221	94,192,414
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	2,032,017	1,009,038
Derivatives Liabilities, Notional	123,993,234	85,874,684
Currency forwards \| Derivative instruments designed as hedging instruments:
Fair value of derivative instruments on a gross basis
Derivatives Liabilities, Fair Value	(514)
Derivatives Liabilities, Notional	16,115
Options
Fair value of derivative instruments on a gross basis
Derivatives Assets, Fair Value	7,045	141
Derivatives Liabilities, Fair Value	5,839	80
Options \| Receivables from broker dealers and clearing organizations
Fair value of derivative instruments on a gross basis
Derivatives Assets, Fair Value	7,045	141
Derivatives Assets, Notional	682,369	6,844
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	5,839	80
Derivatives Liabilities, Notional	681,147	4,486
Treasury futures \| Receivables from broker dealers and clearing organizations
Fair value of derivative instruments on a gross basis
Derivatives Assets, Fair Value	73	31
Derivatives Assets, Notional	$ 44,395	$ 5,730

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
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-URI http://asc.fasb.org/extlink&oid=6519104

Reference 3: http://www.xbrl.org/2003/role/presentationRef
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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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