Derivative Financial Instruments - Gain (Loss) Recognized (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Derivative Instruments Not Designated as Hedging Instruments [Member] | Trading Revenues, Net [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative instruments not designated as hedging instruments, Gain (Loss)	$ 90,718		$ 253,761		$ 272,235
Derivative Instruments Not Designated as Hedging Instruments [Member] | Foreign Currency Futures Contracts [Member] | Trading Revenues, Net [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative instruments not designated as hedging instruments, Gain (Loss)	10,535		12,191		19,699
Derivative Instruments Not Designated as Hedging Instruments [Member] | Foreign Currency Forward Contracts [Member] | Investment Income and Other, Net [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative instruments not designated as hedging instruments, Gain (Loss)	526		0		0
Derivative Instruments Not Designated as Hedging Instruments [Member] | Equity Futures Contracts [Member] | Trading Revenues, Net [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative instruments not designated as hedging instruments, Gain (Loss)	25,247		50,073		76,089
Derivative Instruments Not Designated as Hedging Instruments [Member] | Equity Swap Contracts [Member] | Trading Revenues, Net [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative instruments not designated as hedging instruments, Gain (Loss)	5,277		11,736		8,116
Derivative Instruments Not Designated as Hedging Instruments [Member] | Listed Options [Member] | Trading Revenues, Net [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative instruments not designated as hedging instruments, Gain (Loss)	(37,439)	[1]	37,035	[1]	50,446	[1]
Derivative Instruments Not Designated as Hedging Instruments [Member] | Fixed Income Futures Contracts [Member] | Trading Revenues, Net [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative instruments not designated as hedging instruments, Gain (Loss)	31,277		80,511		81,933
Derivative Instruments Not Designated as Hedging Instruments [Member] | Commodity Futures Contracts [Member] | Trading Revenues, Net [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative instruments not designated as hedging instruments, Gain (Loss)	55,295		62,215		35,952
Derivative Instruments Designated as Hedging Instruments [Member] | Foreign Currency Forward Contracts [Member] | Accumulated Other Comprehensive Income [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative instruments designated as hedging instruments	$ 0		$ (3,298)		$ 0

[1] Realized gains and losses on listed equity options relate to the Company’s market making activities in such options. Such market making activities also comprise trading in the underlying equity securities with gains and losses on such securities generally offsetting the gains and losses reported in this table. Gains and losses on such equity securities are also included in Trading revenues, net on the Company’s Consolidated Statements of Operations.