Derivatives, Derivatives Instruments Gain (Loss) in Income Statement (Details) - Not Designated as Hedging Instrument [Member] - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Derivative Instruments, Gain (Loss) Recognized in Income, Net [Abstract]
Brokergage and Trading Revenue	$ 11,171	$ 8,868
Gain (Loss) on Derivatives, Net	(450)	7,138
To-be-announced residential mortgage-backed securities
Derivative Instruments, Gain (Loss) Recognized in Income, Net [Abstract]
Brokergage and Trading Revenue	8,027	7,440
Gain (Loss) on Derivatives, Net	0	0
Interest rate swaps
Derivative Instruments, Gain (Loss) Recognized in Income, Net [Abstract]
Brokergage and Trading Revenue	459	325
Gain (Loss) on Derivatives, Net	0	0
Energy contracts
Derivative Instruments, Gain (Loss) Recognized in Income, Net [Abstract]
Brokergage and Trading Revenue	2,873	696
Gain (Loss) on Derivatives, Net	0	0
Agricultural contracts
Derivative Instruments, Gain (Loss) Recognized in Income, Net [Abstract]
Brokergage and Trading Revenue	9	29
Gain (Loss) on Derivatives, Net	0	0
Foreign exchange contracts
Derivative Instruments, Gain (Loss) Recognized in Income, Net [Abstract]
Brokergage and Trading Revenue	270	378
Gain (Loss) on Derivatives, Net	0	0
Equity option contracts
Derivative Instruments, Gain (Loss) Recognized in Income, Net [Abstract]
Brokergage and Trading Revenue	0	0
Gain (Loss) on Derivatives, Net	0	0
Total customer risk management programs
Derivative Instruments, Gain (Loss) Recognized in Income, Net [Abstract]
Brokergage and Trading Revenue	11,638	8,868
Gain (Loss) on Derivatives, Net	0	0
Internal risk management programs
Derivative Instruments, Gain (Loss) Recognized in Income, Net [Abstract]
Brokergage and Trading Revenue	(467)	0
Gain (Loss) on Derivatives, Net	$ (450)	$ 7,138