Financial Assets and Liabilities - Netting of Certain Financial Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Securities loaned
Gross Amounts of Recognized Liabilities	$ 754,687	$ 222,203
Net Amounts of Liabilities Presented in the Consolidated Statement of Financial Condition	754,687	222,203
Gross Amounts Not Offset in the Consolidated Statement of Financial Condition
Financial instruments	(737,731)	(221,792)
Cash collateral received	(10,776)
Net Amount	6,180	411
Securities sold under agreements to repurchase
Gross Amounts of Recognized Liabilities	390,642
Net Amounts of Liabilities Presented in the Consolidated Statement of Financial Condition	390,642
Gross Amounts Not Offset in the Consolidated Statement of Financial Condition
Financial instruments	(390,642)
Total
Gross Amounts of Recognized Liabilities	3,183,185	1,231,321
Gross Amounts Offset in the Consolidated Statement of Financial Condition	(2,024,991)	(1,009,038)
Net Amounts of Liabilities Presented in the Consolidated Statement of Financial Condition	1,158,194	222,283
Gross Amounts Not Offset in the Consolidated Statement of Financial Condition
Financial instruments	(1,128,429)	(221,872)
Cash collateral received	(10,776)
Net Amount	18,989	411
Currency forwards
Trading liabilities, at fair value:
Gross Amounts of Recognized Liabilities	2,032,017	1,009,038
Gross Amounts Offset in the Consolidated Statement of Financial Condition	(2,024,991)	(1,009,038)
Net Amounts of Assets Presented in the Consolidated Statement of Financial Condition	7,026
Gross Amounts Not Offset in the Consolidated Statement of Financial Condition
Net Amount	7,026
Options
Trading liabilities, at fair value:
Gross Amounts of Recognized Liabilities	5,839	80
Net Amounts of Assets Presented in the Consolidated Statement of Financial Condition	5,839	80
Gross Amounts Not Offset in the Consolidated Statement of Financial Condition
Financial instruments	(56)	$ (80)
Net Amount	$ 5,783