Financial Assets and Liabilities - Netting of Certain Financial Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Securities borrowed
Gross Amounts of Recognized Assets	$ 453,296	$ 484,934
Net Amounts of Assets Presented in the Consolidated Statement of Financial Condition	453,296	484,934
Gross Amounts Not Offset in the Consolidated Statement of Financial Condition
Financial instruments	(443,659)	(477,559)
Cash collateral received	(281)
Net Amount	9,356	7,375
Securities purchased under agreements to resell
Gross Amounts of Recognized Assets	14,981	31,463
Net Amounts of Assets Presented in the Consolidated Statement of Financial Condition	14,981	31,463
Gross Amounts Not Offset in the Consolidated Statement of Financial Condition
Financial instruments	(14,981)	(31,463)
Total
Gross Amounts of Recognized Assets	1,264,191	2,146,355
Gross Amounts Offset in the Consolidated Statement of Financial Condition	(785,855)	(1,629,629)
Net Amounts of Assets Presented in the Consolidated Statement of Financial Condition	478,336	516,726
Gross Amounts Not Offset in the Consolidated Statement of Financial Condition
Financial instruments	(459,090)	(509,098)
Cash collateral received	(281)
Net Amount	18,965	7,628
Currency forwards
Trading assets, at fair value
Gross Amounts of Recognized Assets	795,435	1,629,637
Gross Amounts Offset in the Consolidated Statement of Financial Condition	(785,855)	(1,629,629)
Net Amounts of Assets Presented in the Consolidated Statement of Financial Condition	9,580	8
Gross Amounts Not Offset in the Consolidated Statement of Financial Condition
Net Amount	9,580	8
Options
Trading assets, at fair value
Gross Amounts of Recognized Assets	168	321
Net Amounts of Assets Presented in the Consolidated Statement of Financial Condition	168	321
Gross Amounts Not Offset in the Consolidated Statement of Financial Condition
Financial instruments	(139)	(76)
Net Amount	29	$ 245
Interest rate swaps
Trading assets, at fair value
Gross Amounts of Recognized Assets	311
Net Amounts of Assets Presented in the Consolidated Statement of Financial Condition	311
Gross Amounts Not Offset in the Consolidated Statement of Financial Condition
Financial instruments	$ (311)