Financial Assets and Liabilities - Netting of Certain Financial Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Securities borrowed
Gross Amounts of Recognized Assets	$ 1,399,684	$ 1,471,172
Net Amounts of Assets Presented in the Consolidated Statement of Financial Condition	1,399,684	1,471,172
Gross Amounts Not Offset In the Consolidated Statement of Financial Condition
Financial Instruments	(1,361,635)	(1,418,672)
Cash Collateral Received	(8,822)	(13,318)
Net Amount	29,227	39,182
Securities purchased under agreements to resell
Gross Amounts of Recognized Assets	15,475
Net Amounts of Assets Presented in the Consolidated Statement of Financial Condition	15,475	0
Gross Amounts Not Offset In the Consolidated Statement of Financial Condition
Financial Instruments	(15,475)
Cash Collateral Received	0
Net Amount	0
Total
Gross Amounts of Recognized Assets	4,219,431	3,523,704
Gross Amounts Offset in the Consolidated Statement of Financial Condition	(2,790,242)	(2,027,697)
Net Amounts of Assets Presented in the Consolidated Statement of Financial Condition	1,429,189	1,496,007
Gross Amounts Not Offset In the Consolidated Statement of Financial Condition
Financial Instruments	(1,389,009)	(1,418,717)
Cash Collateral Received	(8,822)	(13,318)
Net Amount	31,358	63,972
Currency forwards
Trading assets, at fair value:
Gross Amounts of Recognized Assets	2,792,373	2,045,487
Gross Amounts Offset in the Consolidated Statement of Financial Condition	(2,790,242)	(2,027,697)
Net Amounts of Assets Presented in the Consolidated Statement of Financial Condition	2,131	17,790
Gross Amounts Not Offset In the Consolidated Statement of Financial Condition
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Amount	2,131	17,790
Options
Trading assets, at fair value:
Gross Amounts of Recognized Assets	11,899	7,045
Gross Amounts Offset in the Consolidated Statement of Financial Condition	0	0
Net Amounts of Assets Presented in the Consolidated Statement of Financial Condition	11,899	7,045
Gross Amounts Not Offset In the Consolidated Statement of Financial Condition
Financial Instruments	(11,899)	(45)
Cash Collateral Received	0	0
Net Amount	$ 0	$ 7,000