Derivatives, Derivatives Instruments Gain (Loss) in Income Statement (Details) - Not Designated as Hedging Instrument [Member] - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Derivative Instruments, Gain (Loss) Recognized in Income, Net [Abstract]
Brokergage and Trading Revenue	$ 12,151	$ 9,093	$ 34,510	$ 31,080
Gain (Loss) on Derivatives, Net	2,226	1,283	20,130	1,162
To-be-announced residential mortgage-backed securities
Derivative Instruments, Gain (Loss) Recognized in Income, Net [Abstract]
Brokergage and Trading Revenue	11,584	7,914	28,886	25,942
Gain (Loss) on Derivatives, Net	0	0	0	0
Interest rate swaps
Derivative Instruments, Gain (Loss) Recognized in Income, Net [Abstract]
Brokergage and Trading Revenue	710	411	1,758	1,495
Gain (Loss) on Derivatives, Net	0	0	0	0
Energy contracts
Derivative Instruments, Gain (Loss) Recognized in Income, Net [Abstract]
Brokergage and Trading Revenue	1,222	771	4,667	3,138
Gain (Loss) on Derivatives, Net	0	0	0	0
Agricultural contracts
Derivative Instruments, Gain (Loss) Recognized in Income, Net [Abstract]
Brokergage and Trading Revenue	25	44	86	86
Gain (Loss) on Derivatives, Net	0	0	0	0
Foreign exchange contracts
Derivative Instruments, Gain (Loss) Recognized in Income, Net [Abstract]
Brokergage and Trading Revenue	218	152	730	618
Gain (Loss) on Derivatives, Net	0	0	0	0
Equity option contracts
Derivative Instruments, Gain (Loss) Recognized in Income, Net [Abstract]
Brokergage and Trading Revenue	0	0	0	0
Gain (Loss) on Derivatives, Net	0	0	0	0
Total customer risk management programs
Derivative Instruments, Gain (Loss) Recognized in Income, Net [Abstract]
Brokergage and Trading Revenue	13,759	9,292	36,127	31,279
Gain (Loss) on Derivatives, Net	0	0	0	0
Internal risk management programs
Derivative Instruments, Gain (Loss) Recognized in Income, Net [Abstract]
Brokergage and Trading Revenue	(1,608)	(199)	(1,617)	(199)
Gain (Loss) on Derivatives, Net	$ 2,226	$ 1,283	$ 20,130	$ 1,162