Derivative Instruments - Fair Value of Derivative Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Equities futures | Derivative instruments not designated as hedging instruments | Receivables from broker-dealers and clearing organizations
Derivatives Assets
Fair Value	$ (1,366)	$ (15,382)
Notional	4,502,017	2,891,606
Equities futures | Derivative instruments not designated as hedging instruments | Payables to broker-dealers and clearing organizations
Derivatives Liabilities
Fair Value	751	468
Notional	83,803	106,487
Commodity futures | Derivative instruments not designated as hedging instruments | Receivables from broker-dealers and clearing organizations
Derivatives Assets
Fair Value	40,656	69,235
Notional	7,758,974	11,595,215
Commodity futures | Derivative instruments not designated as hedging instruments | Payables to broker-dealers and clearing organizations
Derivatives Liabilities
Fair Value	(45,175)	(375)
Notional	3,604,979	54,782
Currency futures | Derivative instruments not designated as hedging instruments | Receivables from broker-dealers and clearing organizations
Derivatives Assets
Fair Value	(2,860)	(9,432)
Notional	1,116,246	3,756,914
Currency futures | Derivative instruments not designated as hedging instruments | Payables to broker-dealers and clearing organizations
Derivatives Liabilities
Fair Value	(23,223)	(30,643)
Notional	6,594,991	6,239,725
Fixed income futures | Derivative instruments not designated as hedging instruments | Receivables from broker-dealers and clearing organizations
Derivatives Assets
Fair Value	47	(28)
Notional	155,697	18,694
Fixed income futures | Derivative instruments not designated as hedging instruments | Payables to broker-dealers and clearing organizations
Derivatives Liabilities
Fair Value	94	93
Notional	190,938	8,591
Options
Derivatives Assets
Fair Value	8,538	11,899
Derivatives Liabilities
Fair Value	3,087	11,051
Options | Derivative instruments not designated as hedging instruments | Financial instruments owned
Derivatives Assets
Fair Value	8,538	11,899
Notional	442,808	659,101
Options | Derivative instruments not designated as hedging instruments | Financial instruments sold, not yet purchased
Derivatives Liabilities
Fair Value	3,087	11,051
Notional	436,422	608,756
Currency forwards
Derivatives Assets
Fair Value	242,552	2,792,373
Derivatives Liabilities
Fair Value	196,554	2,720,749
Currency forwards | Derivative instruments not designated as hedging instruments | Financial instruments owned
Derivatives Assets
Fair Value	242,552	2,792,373
Notional	24,369,818	171,288,432
Currency forwards | Derivative instruments not designated as hedging instruments | Financial instruments sold, not yet purchased
Derivatives Liabilities
Fair Value	196,554	2,720,749
Notional	24,346,818	171,252,224
Currency forwards | Derivative instruments designated as hedging instruments | Financial instruments sold, not yet purchased
Derivatives Liabilities
Fair Value	0	(792)
Notional	0	13,501
Interest rate swap | Derivative instruments not designated as hedging instruments | Other assets
Derivatives Assets
Fair Value	8,976	0
Notional	$ 525,000	$ 0