Derivative Instruments - Fair Value of Derivative Instruments (Details) - USD ($) $ in Thousands	Jun. 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	$ 1,269	$ 2,403
Derivatives Assets, Notional	741,005	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	(21)	(43)
Derivatives Liabilities, Notional	70,599	62,417
Commodity futures \| Receivables from broker dealers and clearing organizations
Fair value of derivative instruments on a gross basis
Derivatives Assets, Fair Value	(25,096)	13,964
Derivatives Assets, Notional	16,391,955	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	(416)	2,842
Derivatives Liabilities, Notional	30,239	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	(979)	1,591
Derivatives Assets, Notional	6,247,397	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	512	(6,282)
Derivatives Liabilities, Notional	1,935,204	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	(98)	31
Derivatives Assets, Notional	4,024	5,730
Currency forwards
Fair value of derivative instruments on a gross basis
Derivatives Assets, Fair Value	2,513,850	1,147,261
Derivatives Liabilities, Fair Value	2,508,096	1,009,038
Currency forwards \| Financial instruments owned
Fair value of derivative instruments on a gross basis
Derivatives Assets, Fair Value	2,513,850	1,147,261
Derivatives Assets, Notional	139,565,292	94,192,414
Currency forwards \| Financial instruments sold, not yet purchased
Fair value of derivative instruments on a gross basis
Derivatives Liabilities, Fair Value	2,508,096	1,009,038
Derivatives Liabilities, Notional	139,562,415	85,874,684
Options
Fair value of derivative instruments on a gross basis
Derivatives Assets, Fair Value	101	141
Derivatives Liabilities, Fair Value	132	80
Derivatives Liabilities, Notional	11,418	4,486
Options \| Receivables from broker dealers and clearing organizations
Fair value of derivative instruments on a gross basis
Derivatives Assets, Fair Value	101	141
Derivatives Assets, Notional	$ 9,879	$ 6,844

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

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
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-Section 50
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-URI http://asc.fasb.org/extlink&oid=64837297&loc=SL5580258-113959

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 20
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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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Nominal or face amount used to calculate payments on the derivative liability.

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