Financial Assets and Liabilities - Netting of Certain Financial Liabilities (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Securities loaned
Gross Amounts of Recognized Liabilities	$ 582,915	$ 222,203
Net Amounts of Liabilities Presented in the Consolidated Statement of Financial Condition	582,915	222,203
Gross Amounts Not Offset in the Consolidated Statement of Financial Condition
Financial instruments	(574,885)	(221,792)
Net Amount	8,030	411
Securities sold under agreements to repurchase
Gross Amounts of Recognized Liabilities	620,887
Net Amounts of Liabilities Presented in the Consolidated Statement of Financial Condition	620,887
Gross Amounts Not Offset in the Consolidated Statement of Financial Condition
Financial instruments	(620,887)
Total
Gross Amounts of Recognized Liabilities	4,706,994	1,231,321
Gross Amounts Offset in the Consolidated Statement of Financial Condition	(3,492,847)	(1,009,038)
Net Amounts of Liabilities Presented in the Consolidated Statement of Financial Condition	1,214,147	222,283
Gross Amounts Not Offset in the Consolidated Statement of Financial Condition
Financial instruments	(1,196,277)	(221,872)
Net Amount	17,870	411
Receivables from broker dealers and clearing organizations | Quoted Prices in Active Markets for Identical Assets (Level 1)
Gross Amounts Not Offset in the Consolidated Statement of Financial Condition
Net variation margin on long and short futures contracts	89,300	18,000
Payables to broker dealers and clearing organizations | Quoted Prices in Active Markets for Identical Assets (Level 1)
Gross Amounts Not Offset in the Consolidated Statement of Financial Condition
Net variation margin on long and short futures contracts	(50,400)	(3,500)
Currency forwards
Trading liabilities, at fair value:
Gross Amounts of Recognized Liabilities	3,492,928	1,009,038
Gross Amounts Offset in the Consolidated Statement of Financial Condition	(3,492,847)	(1,009,038)
Net Amounts of Assets Presented in the Consolidated Statement of Financial Condition	81
Gross Amounts Not Offset in the Consolidated Statement of Financial Condition
Net Amount	81
Options
Trading liabilities, at fair value:
Gross Amounts of Recognized Liabilities	10,264	80
Net Amounts of Assets Presented in the Consolidated Statement of Financial Condition	10,264	80
Gross Amounts Not Offset in the Consolidated Statement of Financial Condition
Financial instruments	(505)	$ (80)
Net Amount	$ 9,759