Derivative Instruments - Fair Value of Derivative Instruments (Details) - USD ($) $ in Thousands	Sep. 30, 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	$ (2,729)	$ 2,403
Derivatives Assets, Notional	951,330	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	(731)	(43)
Derivatives Liabilities, Notional	107,774	62,417
Commodity futures \| Receivables from broker dealers and clearing organizations
Fair value of derivative instruments on a gross basis
Derivatives Assets, Fair Value	50,258	13,964
Derivatives Assets, Notional	18,078,494	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	(35,301)	2,842
Derivatives Liabilities, Notional	724,829	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	41,826	1,591
Derivatives Assets, Notional	3,789,287	3,264,093
Currency futures \| Payables to broker dealers and clearing organizations
Fair value of derivative instruments on a gross basis
Derivatives Liabilities, Fair Value	(14,425)	(6,282)
Derivatives Liabilities, Notional	2,988,821	1,137,908
Fixed Income futures \| Receivables from broker dealers and clearing organizations
Fair value of derivative instruments on a gross basis
Derivatives Assets, Fair Value	(54)	31
Derivatives Assets, Notional	32,723	5,730
Currency forwards
Fair value of derivative instruments on a gross basis
Derivatives Assets, Fair Value	3,459,340	1,147,261
Derivatives Liabilities, Fair Value	3,492,928	1,009,038
Currency forwards \| Financial instruments owned
Fair value of derivative instruments on a gross basis
Derivatives Assets, Fair Value	3,459,340	1,147,261
Derivatives Assets, Notional	159,559,280	94,192,414
Currency forwards \| Financial instruments sold, not yet purchased
Fair value of derivative instruments on a gross basis
Derivatives Liabilities, Fair Value	3,492,928	1,009,038
Derivatives Liabilities, Notional	159,585,203	85,874,684
Options
Fair value of derivative instruments on a gross basis
Derivatives Assets, Fair Value	11,220	141
Derivatives Liabilities, Fair Value	10,264	80
Derivatives Liabilities, Notional	1,108,920	4,486
Options \| Receivables from broker dealers and clearing organizations
Fair value of derivative instruments on a gross basis
Derivatives Assets, Fair Value	11,220	141
Derivatives Assets, Notional	1,122,415	$ 6,844
Treasury futures \| Payables to broker dealers and clearing organizations
Fair value of derivative instruments on a gross basis
Derivatives Liabilities, Fair Value	11
Derivatives Liabilities, Notional	$ 7,906

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

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+ References

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

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