Fair Value of Financial Instruments - Fair Value of Gross amounts offset (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Offsetting Liabilities [Line Items]
Gross Amounts of Recognized, Asset	$ 1,659,148	$ 144,566
Gross Amounts Offset in the Statements of Financial Condition, Assets	0	0
Net Amounts of Assets Presented in the Statements of Financial Condition, Assets	1,659,148	144,566
Gross Amounts Not Offset in the Statement of Financial Position, Financial Instruments, Assets	1,329,508	50,717
Gross Amounts Not Offset in the Statement of Financial Position, Cash Collateral Received, Assets	0	0
Net Amount, Assets	329,640	93,849
Gross Amounts of Recognized Derivatives, Liability	1,373,633	69,757
Gross Amounts of Recognized Derivatives, Liability	0	0
Net Amounts of Assets Presented in the Statements of Financial Condition, Liability	1,373,633	69,757
Gross Amounts Not Offset in the Statement of Financial Condition, Financial Instruments, Liability	1,128,288	0
Gross Amounts Not Offset in the Statement of Financial Position, Cash Collateral Received, Liability	5,064	1,393
Net Amount, Liability	240,281	68,364
Financial instruments sold under agreements to repurchase	595,029	0
Listed equity options [Member]
Offsetting Liabilities [Line Items]
Gross Amounts of Recognized, Asset	288,227	92,305
Gross Amounts Offset in the Statements of Financial Condition, Assets	0	0
Net Amounts of Assets Presented in the Statements of Financial Condition, Assets	288,227	92,305
Gross Amounts Not Offset in the Statement of Financial Position, Financial Instruments, Assets	0	0
Gross Amounts Not Offset in the Statement of Financial Position, Cash Collateral Received, Assets	0	0
Net Amount, Assets	288,227	92,305
Gross Amounts of Recognized Derivatives, Liability	229,722	69,757
Gross Amounts of Recognized Derivatives, Liability	0	0
Net Amounts of Assets Presented in the Statements of Financial Condition, Liability	229,722	69,757
Gross Amounts Not Offset in the Statement of Financial Condition, Financial Instruments, Liability	0	0
Gross Amounts Not Offset in the Statement of Financial Position, Cash Collateral Received, Liability	5,064	1,393
Net Amount, Liability	224,658	68,364
Securities borrowed [Member]
Offsetting Liabilities [Line Items]
Gross Amounts of Recognized, Asset	1,370,921	52,261
Gross Amounts Offset in the Statements of Financial Condition, Assets	0	0
Net Amounts of Assets Presented in the Statements of Financial Condition, Assets	1,370,921	52,261
Gross Amounts Not Offset in the Statement of Financial Position, Financial Instruments, Assets	1,329,508	50,717
Gross Amounts Not Offset in the Statement of Financial Position, Cash Collateral Received, Assets	0
Net Amount, Assets	41,413	1,544
Securities loaned [Member]
Offsetting Liabilities [Line Items]
Gross Amounts of Recognized Derivatives, Liability	543,451
Gross Amounts of Recognized Derivatives, Liability	0
Net Amounts of Assets Presented in the Statements of Financial Condition, Liability	543,451
Gross Amounts Not Offset in the Statement of Financial Condition, Financial Instruments, Liability	533,266
Gross Amounts Not Offset in the Statement of Financial Position, Cash Collateral Received, Liability	0
Net Amount, Liability	10,185
Repurchase Agreements [Member]
Offsetting Liabilities [Line Items]
Gross Amounts of Recognized Derivatives, Liability	595,029
Gross Amounts of Recognized Derivatives, Liability	0
Net Amounts of Assets Presented in the Statements of Financial Condition, Liability	595,029
Gross Amounts Not Offset in the Statement of Financial Condition, Financial Instruments, Liability	595,022
Gross Amounts Not Offset in the Statement of Financial Position, Cash Collateral Received, Liability	0
Net Amount, Liability	7
Foreign currency forward contracts [Member]
Offsetting Liabilities [Line Items]
Gross Amounts of Recognized Derivatives, Liability	5,431
Gross Amounts of Recognized Derivatives, Liability	0
Net Amounts of Assets Presented in the Statements of Financial Condition, Liability	5,431
Gross Amounts Not Offset in the Statement of Financial Condition, Financial Instruments, Liability	0
Gross Amounts Not Offset in the Statement of Financial Position, Cash Collateral Received, Liability	0
Net Amount, Liability	$ 5,431