Fair Value of Financial Instruments - Fair Value of Gross amounts offset (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013		Dec. 31, 2012
Offsetting Liabilities [Line Items]
Gross Amounts of Recognized, Asset	$ 1,348,223		$ 1,210,811
Gross Amounts Offset in the Statements of Financial Condition, Assets	0		0
Net Amounts of Assets Presented in the Statements of Financial Condition, Assets	1,348,223		1,210,811
Gross Amounts Not Offset in the Statement of Financial Position, Financial Instruments, Assets	1,126,224		991,302
Gross Amounts Not Offset in the Statement of Financial Position, Cash Collateral Received, Assets	0		0
Net Amount, Assets	221,999		219,509
Gross Amounts of Recognized Derivatives, Liability	1,320,303		1,130,957
Gross Amounts of Recognized Derivatives, Liability	0		0
Net Amounts of Assets Presented in the Statements of Financial Condition, Liability	1,320,303		1,130,957
Gross Amounts Not Offset in the Statement of Financial Condition, Financial Instruments, Liability	1,178,077		967,032
Gross Amounts Not Offset in the Statement of Financial Position, Cash Collateral Received, Liability	5,499		8,439
Net Amount, Liability	136,727		155,486
Financial instruments sold under agreements to repurchase	545,000		355,000
Listed equity options [Member]
Offsetting Liabilities [Line Items]
Gross Amounts of Recognized, Asset	188,166		202,091
Gross Amounts Offset in the Statements of Financial Condition, Assets	0		0
Net Amounts of Assets Presented in the Statements of Financial Condition, Assets	188,166		202,091
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	188,166		202,091
Gross Amounts of Recognized Derivatives, Liability	132,161		155,942
Gross Amounts of Recognized Derivatives, Liability	0		0
Net Amounts of Assets Presented in the Statements of Financial Condition, Liability	132,161		155,942
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	5,499		8,439
Net Amount, Liability	126,662		147,503
Foreign currency forward contracts [Member]
Offsetting Liabilities [Line Items]
Gross Amounts of Recognized, Asset	1,076
Gross Amounts Offset in the Statements of Financial Condition, Assets	0
Net Amounts of Assets Presented in the Statements of Financial Condition, Assets	1,076
Gross Amounts Not Offset in the Statement of Financial Position, Financial Instruments, Assets	0
Gross Amounts Not Offset in the Statement of Financial Position, Cash Collateral Received, Assets	0
Net Amount, Assets	1,076
Gross Amounts of Recognized Derivatives, Liability			4,446
Gross Amounts of Recognized Derivatives, Liability			0
Net Amounts of Assets Presented in the Statements of Financial Condition, Liability			4,446
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			4,446
Securities borrowed [Member]
Offsetting Liabilities [Line Items]
Gross Amounts of Recognized, Asset	1,158,981		1,008,720
Gross Amounts Offset in the Statements of Financial Condition, Assets	0		0
Net Amounts of Assets Presented in the Statements of Financial Condition, Assets	1,158,981		1,008,720
Gross Amounts Not Offset in the Statement of Financial Position, Financial Instruments, Assets	1,126,224		991,302
Gross Amounts Not Offset in the Statement of Financial Position, Cash Collateral Received, Assets	0
Net Amount, Assets	32,757		17,418
Securities loaned [Member]
Offsetting Liabilities [Line Items]
Gross Amounts of Recognized Derivatives, Liability	626,891		504,082
Gross Amounts of Recognized Derivatives, Liability	0		0
Net Amounts of Assets Presented in the Statements of Financial Condition, Liability	626,891		504,082
Gross Amounts Not Offset in the Statement of Financial Condition, Financial Instruments, Liability	616,826		500,545
Gross Amounts Not Offset in the Statement of Financial Position, Cash Collateral Received, Liability	0		0
Net Amount, Liability	10,065		3,537
Repurchase Agreements [Member]
Offsetting Liabilities [Line Items]
Gross Amounts of Recognized Derivatives, Liability	561,251	[1]	466,487	[1]
Gross Amounts of Recognized Derivatives, Liability	0	[1]	0	[1]
Net Amounts of Assets Presented in the Statements of Financial Condition, Liability	561,251	[1]	466,487	[1]
Gross Amounts Not Offset in the Statement of Financial Condition, Financial Instruments, Liability	561,251	[1]	466,487	[1]
Gross Amounts Not Offset in the Statement of Financial Position, Cash Collateral Received, Liability	0	[1]	0	[1]
Net Amount, Liability	0	[1]	0	[1]
Financial instruments sold under agreements to repurchase	$ 16,300		$ 111,500

[1] Financial instruments sold under agreements to repurchase includes $16.3 million and $111.5 million at June 30, 2013 and December 31, 2012, respectively, recorded within Liabilities within discontinued operations and Liabilities of business held for sale, respectively.