Financial Assets and Liabilities - Netting of Certain Financial Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Securities loaned
Gross Amounts of Recognized Liabilities	$ 1,130,039	$ 754,687
Net Amounts of Assets Presented in the Consolidated Statement of Financial Condition	1,130,039	754,687
Gross Amounts Not Offset In the Consolidated Statement of Financial Condition
Financial Instruments	(1,108,461)	(737,731)
Cash Collateral Pledged	(8,822)	(10,776)
Net Amount	12,756	6,180
Securities sold under agreements to repurchase
Gross Amounts of Recognized Liabilities	281,861	390,642
Net Amounts of Assets Presented in the Consolidated Statement of Financial Condition	281,861	390,642
Gross Amounts Not Offset In the Consolidated Statement of Financial Condition
Financial Instruments	(281,861)	(390,642)
Cash Collateral Pledged	0	0
Net Amount	0	0
Total
Gross Amounts of Recognized Liabilities	4,143,700	3,183,185
Gross Amounts Offset in the Consolidated Statement of Financial Condition	(2,719,954)	(2,024,991)
Net Amounts of Assets Presented in the Consolidated Statement of Financial Condition	1,423,746	1,158,194
Gross Amounts Not Offset In the Consolidated Statement of Financial Condition
Financial Instruments	(1,401,373)	(1,128,429)
Cash Collateral Pledged	(9,614)	(10,776)
Net Amount	12,759	18,989
Currency forwards
Trading liabilities, at fair value:
Gross Amounts of Recognized Liabilities	2,720,749	2,032,017
Gross Amounts Offset in the Consolidated Statement of Financial Condition	(2,719,954)	(2,024,991)
Net Amounts of Assets Presented in the Consolidated Statement of Financial Condition	795	7,026
Gross Amounts Not Offset In the Consolidated Statement of Financial Condition
Financial Instruments	0	0
Cash Collateral Pledged	(792)
Net Amount	3	7,026
Options
Trading liabilities, at fair value:
Gross Amounts of Recognized Liabilities	11,051	5,839
Gross Amounts Offset in the Consolidated Statement of Financial Condition	0	0
Net Amounts of Assets Presented in the Consolidated Statement of Financial Condition	11,051	5,839
Gross Amounts Not Offset In the Consolidated Statement of Financial Condition
Financial Instruments	(11,051)	(56)
Cash Collateral Pledged	0	0
Net Amount	$ 0	$ 5,783