Collateralized Transactions and Assets and Liabilities Subject to Netting - Gross Amounts Offset (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013
Offsetting Liabilities [Line Items]
Gross Amounts of Recognized, Assets	$ 1,823,018		$ 1,736,983
Gross Amounts Offset in the Statements of Financial Condition, Assets	24,436		10,870
Net Amounts Presented in the Statements of Financial Condition, Assets	1,798,582		1,726,113
Gross Amounts Not Offset in the Statement of Financial Condition, Available Collateral, Assets	1,591,619	[1]	1,340,359	[1]
Gross Amounts Not Offset in the Statement of Financial Condition, Counterparty Netting, Assets	15,782	[2]	10,110	[2]
Net Amount, Assets	191,181		375,644
Gross Amounts of Recognized Derivatives, Liabilities	1,781,118		1,643,258
Gross Amounts of Recognized Derivatives, Liabilities	24,436		11,764
Net Amounts Presented in the Statements of Financial Condition, Liabilities	1,756,682		1,631,494
Gross Amounts Not Offset in the Statement of Financial Condition, Available Collateral, Liabilities	1,615,949	[1]	1,357,786	[1]
Gross Amounts Not Offset in the Statement of Financial Condition, Counterparty Netting, Liabilities	33,141	[2]	21,034	[2]
Net Amount, Liabilities	107,592		252,674
Listed Options [Member]
Offsetting Liabilities [Line Items]
Gross Amounts of Recognized, Assets	144,586		339,798
Gross Amounts Offset in the Statements of Financial Condition, Assets	0		0
Net Amounts Presented in the Statements of Financial Condition, Assets	144,586		339,798
Gross Amounts Not Offset in the Statement of Financial Condition, Available Collateral, Assets	0	[1]	0	[1]
Gross Amounts Not Offset in the Statement of Financial Condition, Counterparty Netting, Assets	0	[2]	0	[2]
Net Amount, Assets	144,586		339,798
Gross Amounts of Recognized Derivatives, Liabilities	115,362		252,282
Gross Amounts of Recognized Derivatives, Liabilities	0		0
Net Amounts Presented in the Statements of Financial Condition, Liabilities	115,362		252,282
Gross Amounts Not Offset in the Statement of Financial Condition, Available Collateral, Liabilities	0	[1]	0	[1]
Gross Amounts Not Offset in the Statement of Financial Condition, Counterparty Netting, Liabilities	17,359	[2]	10,924	[2]
Net Amount, Liabilities	98,003		241,358
Securities Borrowed [Member]
Offsetting Liabilities [Line Items]
Gross Amounts of Recognized, Assets	1,632,062		1,357,387
Gross Amounts Offset in the Statements of Financial Condition, Assets	0		0
Net Amounts Presented in the Statements of Financial Condition, Assets	1,632,062		1,357,387
Gross Amounts Not Offset in the Statement of Financial Condition, Available Collateral, Assets	1,570,194	[1]	1,316,110	[1]
Gross Amounts Not Offset in the Statement of Financial Condition, Counterparty Netting, Assets	15,782	[2]	10,110	[2]
Net Amount, Assets	46,086		31,167
Receivable from Brokers, Dealers and Clearing Organizations [Member]
Offsetting Liabilities [Line Items]
Gross Amounts of Recognized, Assets	21,545	[3]	24,366	[3]
Gross Amounts Offset in the Statements of Financial Condition, Assets	0	[3]	0	[3]
Net Amounts Presented in the Statements of Financial Condition, Assets	21,545	[3]	24,366	[3]
Gross Amounts Not Offset in the Statement of Financial Condition, Available Collateral, Assets	21,425	[1],[3]	24,249	[1],[3]
Gross Amounts Not Offset in the Statement of Financial Condition, Counterparty Netting, Assets	0	[2],[3]	0	[2],[3]
Net Amount, Assets	120	[3]	117	[3]
Foreign Currency Forward Contracts [Member]
Offsetting Liabilities [Line Items]
Gross Amounts of Recognized, Assets	60		6,913
Gross Amounts Offset in the Statements of Financial Condition, Assets	0		6,501
Net Amounts Presented in the Statements of Financial Condition, Assets	60		412
Gross Amounts Not Offset in the Statement of Financial Condition, Available Collateral, Assets	0	[1]	0	[1]
Gross Amounts Not Offset in the Statement of Financial Condition, Counterparty Netting, Assets	0	[2]	0	[2]
Net Amount, Assets	60		412
Gross Amounts of Recognized Derivatives, Liabilities			11,515
Gross Amounts of Recognized Derivatives, Liabilities			6,501
Net Amounts Presented in the Statements of Financial Condition, Liabilities			5,014
Gross Amounts Not Offset in the Statement of Financial Condition, Available Collateral, Liabilities			0	[1]
Gross Amounts Not Offset in the Statement of Financial Condition, Counterparty Netting, Liabilities			0	[2]
Net Amount, Liabilities			5,014
Swaps [Member]
Offsetting Liabilities [Line Items]
Gross Amounts of Recognized, Assets	85
Gross Amounts Offset in the Statements of Financial Condition, Assets	0
Net Amounts Presented in the Statements of Financial Condition, Assets	85
Gross Amounts Not Offset in the Statement of Financial Condition, Available Collateral, Assets	0	[1]
Gross Amounts Not Offset in the Statement of Financial Condition, Counterparty Netting, Assets	0	[2]
Net Amount, Assets	85
Gross Amounts of Recognized Derivatives, Liabilities			18
Gross Amounts of Recognized Derivatives, Liabilities			0
Net Amounts Presented in the Statements of Financial Condition, Liabilities			18
Gross Amounts Not Offset in the Statement of Financial Condition, Available Collateral, Liabilities			0	[1]
Gross Amounts Not Offset in the Statement of Financial Condition, Counterparty Netting, Liabilities			0	[2]
Net Amount, Liabilities			18
Futures [Member]
Offsetting Liabilities [Line Items]
Gross Amounts of Recognized, Assets	24,680		8,519
Gross Amounts Offset in the Statements of Financial Condition, Assets	24,436		4,369
Net Amounts Presented in the Statements of Financial Condition, Assets	244		4,150
Gross Amounts Not Offset in the Statement of Financial Condition, Available Collateral, Assets	0	[1]	0	[1]
Gross Amounts Not Offset in the Statement of Financial Condition, Counterparty Netting, Assets	0	[2]	0	[2]
Net Amount, Assets	244		4,150
Gross Amounts of Recognized Derivatives, Liabilities	24,436		5,263
Gross Amounts of Recognized Derivatives, Liabilities	24,436		5,263
Net Amounts Presented in the Statements of Financial Condition, Liabilities	0		0
Gross Amounts Not Offset in the Statement of Financial Condition, Available Collateral, Liabilities	0	[1]	0	[1]
Gross Amounts Not Offset in the Statement of Financial Condition, Counterparty Netting, Liabilities	0	[2]	0	[2]
Net Amount, Liabilities	0		0
Securities Loaned [Member]
Offsetting Liabilities [Line Items]
Gross Amounts of Recognized Derivatives, Liabilities	707,744		733,230
Gross Amounts of Recognized Derivatives, Liabilities	0		0
Net Amounts Presented in the Statements of Financial Condition, Liabilities	707,744		733,230
Gross Amounts Not Offset in the Statement of Financial Condition, Available Collateral, Liabilities	682,389	[1]	716,838	[1]
Gross Amounts Not Offset in the Statement of Financial Condition, Counterparty Netting, Liabilities	15,782	[2]	10,110	[2]
Net Amount, Liabilities	9,573		6,282
Financial Instruments Sold Under Agreements to Repurchase [Member}
Offsetting Liabilities [Line Items]
Gross Amounts of Recognized Derivatives, Liabilities	933,576		640,950
Gross Amounts of Recognized Derivatives, Liabilities	0		0
Net Amounts Presented in the Statements of Financial Condition, Liabilities	933,576		640,950
Gross Amounts Not Offset in the Statement of Financial Condition, Available Collateral, Liabilities	933,560	[1]	640,948	[1]
Gross Amounts Not Offset in the Statement of Financial Condition, Counterparty Netting, Liabilities	0	[2]	0	[2]
Net Amount, Liabilities	$ 16		$ 2

[1] Includes securities received or delivered under collateral arrangements with counterparties that could be liquidated in the event of a counterparty default and thus offset against a counterparty's rights and obligations under the respective repurchase agreements or securities borrowing or lending arrangements.

[2] Under master netting agreements with its counterparties, the company has the legal right of offset with a counterparty, which incorporates all of the counterparty's outstanding rights and obligations under the arrangement. These balances reflect additional credit risk mitigation that is avalable by counterparty in the event of a counterparty's default, but which are not netted in the Statement of Financial Condition because other netting provisions under U.S. GAAP are not met.

[3] Represents reverse repurchase agreements at broker dealer.