Financial Assets and Liabilities - Netting of Certain Financial Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Securities borrowed
Gross Amounts of Recognized Assets	$ 1,525,403	$ 220,005
Net Amounts of Assets Presented in the Consolidated Statement of Financial Condition	1,525,403	220,005
Gross Amounts Not Offset in the Statement of Financial Condition
Financial instruments	(1,480,267)	(216,778)
Cash collateral received	(844)	(248)
Net Amount	44,292	2,979
Securities purchased under agreements to resell
Gross Amounts of Recognized Assets	8,249
Net Amounts of Assets Presented in the Consolidated Statement of Financial Condition	8,249
Gross Amounts Not Offset in the Statement of Financial Condition
Financial instruments	(8,188)
Net Amount	61
Total
Gross Amounts of Recognized Assets	5,004,212	1,367,407
Gross Amounts Offset in the Consolidated Statement of Financial Condition	(3,458,283)	(1,140,239)
Net Amounts of Assets Presented in the Consolidated Statement of Financial Condition	1,545,929	227,168
Gross Amounts Not Offset in the Consolidated Statement of Financial Condition
Financial instruments	(1,488,960)	(216,858)
Cash collateral received	(844)	(261)
Net Amount	56,125	10,049
Currency forwards
Trading assets, at fair value
Gross Amounts of Recognized Assets	3,459,340	1,147,261
Gross Amounts Offset in the Consolidated Statement of Financial Condition	(3,458,283)	(1,140,239)
Net Amounts of Assets Presented in the Consolidated Statement of Financial Condition	1,057	7,022
Gross Amounts Not Offset in the Consolidated Statement of Financial Condition
Net Amount	1,057	7,022
Options
Trading assets, at fair value
Gross Amounts of Recognized Assets	11,220	141
Net Amounts of Assets Presented in the Consolidated Statement of Financial Condition	11,220	141
Gross Amounts Not Offset in the Consolidated Statement of Financial Condition
Financial instruments	(505)	(80)
Cash collateral received		(13)
Net Amount	$ 10,715	$ 48