Derivative Instruments - Fair Value of Derivative Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Equities futures \| Receivables from broker dealers and clearing organizations
Fair value of derivative instruments on a gross basis
Derivatives Assets, Fair Value	$ (1,017)	$ 241
Derivatives Assets, Notional	617,398	561,029
Equities futures \| Payables to broker dealers and clearing organizations
Fair value of derivative instruments on a gross basis
Derivatives Liabilities, Fair Value	202	(268)
Derivatives Liabilities, Notional	75,428	122,948
Commodity futures \| Receivables from broker dealers and clearing organizations
Fair value of derivative instruments on a gross basis
Derivatives Assets, Fair Value	(8,431)	42,489
Derivatives Assets, Notional	18,635,531	28,823,081
Commodity futures \| Payables to broker dealers and clearing organizations
Fair value of derivative instruments on a gross basis
Derivatives Liabilities, Fair Value	585	(295)
Derivatives Liabilities, Notional	25,932	15,727
Currency futures \| Receivables from broker dealers and clearing organizations
Fair value of derivative instruments on a gross basis
Derivatives Assets, Fair Value	1,358	3,180
Derivatives Assets, Notional	3,059,575	2,916,222
Currency futures \| Payables to broker dealers and clearing organizations
Fair value of derivative instruments on a gross basis
Derivatives Liabilities, Fair Value	(12,475)	(3,077)
Derivatives Liabilities, Notional	2,570,005	2,123,341
Treasury futures \| Receivables from broker dealers and clearing organizations
Fair value of derivative instruments on a gross basis
Derivatives Assets, Fair Value	30	504
Derivatives Assets, Notional	232,473	857,363
Options
Fair value of derivative instruments on a gross basis
Derivatives Assets, Fair Value	168	321
Derivatives Liabilities, Fair Value	163	79
Options \| Receivables from broker dealers and clearing organizations
Fair value of derivative instruments on a gross basis
Derivatives Assets, Fair Value	168	321
Derivatives Assets, Notional	24,453	39,802
Options \| Financial instruments sold, not yet purchased
Fair value of derivative instruments on a gross basis
Derivatives Liabilities, Fair Value	163	79
Derivatives Liabilities, Notional	25,336	12,913
Currency forwards
Fair value of derivative instruments on a gross basis
Derivatives Assets, Fair Value	795,435	1,629,637
Derivatives Liabilities, Fair Value	746,014	1,645,820
Currency forwards \| Financial instruments owned
Fair value of derivative instruments on a gross basis
Derivatives Assets, Fair Value	795,435	1,629,637
Derivatives Assets, Notional	69,614,513	127,021,198
Currency forwards \| Financial instruments sold, not yet purchased
Fair value of derivative instruments on a gross basis
Derivatives Liabilities, Fair Value	746,014	1,645,820
Derivatives Liabilities, Notional	71,019,047	125,152,639
Interest rate swaps
Fair value of derivative instruments on a gross basis
Derivatives Assets, Fair Value	311
Derivatives Liabilities, Fair Value	319	12
Interest rate swaps \| Financial instruments owned
Fair value of derivative instruments on a gross basis
Derivatives Assets, Fair Value	311
Derivatives Assets, Notional	82,010
Interest rate swaps \| Financial instruments sold, not yet purchased
Fair value of derivative instruments on a gross basis
Derivatives Liabilities, Fair Value	319	12
Derivatives Liabilities, Notional	$ 82,010	$ 164,020

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=35706170&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
-SubTopic 10
-Section 50
-Paragraph 1B
-URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5580258-113959

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4B
-Subparagraph (a),(c)
-URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5624163-113959

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=34720828&loc=SL20225862-175312

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=36459792&loc=SL20226000-175313

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1B
-URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5580258-113959

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
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-URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5579245-113959

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