Derivative Instruments - Fair Value of Derivative Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Equities futures \| Derivative instruments not designated as hedging instruments \| Receivables from broker dealers and clearing organizations
Derivatives, Fair Value [Line Items]
Derivatives Assets, Fair Value	$ (15,382)	$ (505)
Derivatives Assets, Notional	2,891,606	1,985,770
Equities futures \| Derivative instruments not designated as hedging instruments \| Payables to broker dealers and clearing organizations
Derivatives, Fair Value [Line Items]
Derivatives Liabilities, Fair Value	468	(575)
Derivatives Liabilities, Notional	106,487	142,658
Commodity futures \| Derivative instruments not designated as hedging instruments \| Receivables from broker dealers and clearing organizations
Derivatives, Fair Value [Line Items]
Derivatives Assets, Fair Value	69,235	971
Derivatives Assets, Notional	11,595,215	21,231,001
Commodity futures \| Derivative instruments not designated as hedging instruments \| Payables to broker dealers and clearing organizations
Derivatives, Fair Value [Line Items]
Derivatives Liabilities, Fair Value	(375)	(1,602)
Derivatives Liabilities, Notional	54,782	130,042
Currency futures \| Derivative instruments not designated as hedging instruments \| Receivables from broker dealers and clearing organizations
Derivatives, Fair Value [Line Items]
Derivatives Assets, Fair Value	(9,432)	26,548
Derivatives Assets, Notional	3,756,914	3,994,412
Currency futures \| Derivative instruments not designated as hedging instruments \| Payables to broker dealers and clearing organizations
Derivatives, Fair Value [Line Items]
Derivatives Liabilities, Fair Value	(30,643)	(13,947)
Derivatives Liabilities, Notional	6,239,725	7,756,958
Fixed income futures \| Derivative instruments not designated as hedging instruments \| Receivables from broker dealers and clearing organizations
Derivatives, Fair Value [Line Items]
Derivatives Assets, Fair Value	(28)	73
Derivatives Assets, Notional	18,694	44,395
Fixed income futures \| Derivative instruments not designated as hedging instruments \| Payables to broker dealers and clearing organizations
Derivatives, Fair Value [Line Items]
Derivatives Liabilities, Fair Value	93	(1)
Derivatives Liabilities, Notional	8,591	2,584
Options
Derivatives, Fair Value [Line Items]
Derivatives Assets, Fair Value	11,899	7,045
Derivatives Liabilities, Fair Value	11,051	5,839
Options \| Derivative instruments not designated as hedging instruments \| Financial instruments owned
Derivatives, Fair Value [Line Items]
Derivatives Assets, Fair Value	11,899	7,045
Derivatives Assets, Notional	659,101	682,369
Options \| Derivative instruments not designated as hedging instruments \| Financial instruments sold, not yet purchased
Derivatives, Fair Value [Line Items]
Derivatives Liabilities, Fair Value	11,051	5,839
Derivatives Liabilities, Notional	608,756	681,147
Currency forwards
Derivatives, Fair Value [Line Items]
Derivatives Assets, Fair Value	2,792,373	2,045,487
Derivatives Liabilities, Fair Value	2,720,749	2,032,017
Currency forwards \| Derivative instruments not designated as hedging instruments \| Financial instruments owned
Derivatives, Fair Value [Line Items]
Derivatives Assets, Fair Value	2,792,373	2,045,487
Derivatives Assets, Notional	171,288,432	124,000,221
Currency forwards \| Derivative instruments not designated as hedging instruments \| Financial instruments sold, not yet purchased
Derivatives, Fair Value [Line Items]
Derivatives Liabilities, Fair Value	2,720,749	2,032,017
Derivatives Liabilities, Notional	171,252,224	123,993,234
Currency forwards \| Derivative instruments designated as hedging instruments \| Financial instruments sold, not yet purchased
Derivatives, Fair Value [Line Items]
Derivatives Liabilities, Fair Value	(792)	(514)
Derivatives Liabilities, Notional	$ 13,501	$ 16,115

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

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-Publisher FASB
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