Financial Assets and Liabilities - Netting of Certain Financial Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Securities borrowed
Gross Amounts of Recognized Assets	$ 1,471,172	$ 220,005
Net Amounts of Assets Presented in the Consolidated Statement of Financial Condition	1,471,172	220,005
Gross Amounts Not Offset in the Statement of Financial Condition
Financial instruments	(1,418,672)	(216,778)
Cash collateral received	(13,318)	(248)
Net Amount	39,182	2,979
Gross Amounts Not Offset in the Consolidated Statement of Financial Condition
Securities Loaned, Collateral, Right to Reclaim Cash	10,776
Total
Gross Amounts of Recognized Assets	3,523,704	1,367,407
Gross Amounts Offset in the Consolidated Statement of Financial Condition	(2,027,697)	(1,140,239)
Net Amounts of Assets Presented in the Consolidated Statement of Financial Condition	1,496,007	227,168
Gross Amounts Not Offset in the Consolidated Statement of Financial Condition
Financial instruments	(1,418,717)	(216,858)
Cash collateral received	(13,318)	(261)
Net Amount	63,972	10,049
Currency forwards
Trading assets, at fair value
Gross Amounts of Recognized Assets	2,045,487	1,147,261
Gross Amounts Offset in the Consolidated Statement of Financial Condition	(2,027,697)	(1,140,239)
Net Amounts of Assets Presented in the Consolidated Statement of Financial Condition	17,790	7,022
Gross Amounts Not Offset in the Consolidated Statement of Financial Condition
Net Amount	17,790	7,022
Options
Trading assets, at fair value
Gross Amounts of Recognized Assets	7,045	141
Net Amounts of Assets Presented in the Consolidated Statement of Financial Condition	7,045	141
Gross Amounts Not Offset in the Consolidated Statement of Financial Condition
Financial instruments	(45)	(80)
Cash collateral received		(13)
Net Amount	$ 7,000	$ 48