Derivatives, Derivatives Instruments Gain (Loss) in Income Statement (Details) - Not Designated as Hedging Instrument [Member] - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Derivative Instruments, Gain (Loss) Recognized in Income, Net [Abstract]
Brokergage and Trading Revenue	$ 8,815	$ 18,122	$ 17,838	$ 29,293
Gain (Loss) on Derivatives, Net	(3,057)	3,241	(8,742)	2,791
To-be-announced residential mortgage-backed securities
Derivative Instruments, Gain (Loss) Recognized in Income, Net [Abstract]
Brokergage and Trading Revenue	7,586	9,205	14,405	17,232
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	683	665	1,439	1,124
Gain (Loss) on Derivatives, Net	0	0	0	0
Energy contracts
Derivative Instruments, Gain (Loss) Recognized in Income, Net [Abstract]
Brokergage and Trading Revenue	1,416	1,666	4,556	4,539
Gain (Loss) on Derivatives, Net	0	0	0	0
Agricultural contracts
Derivative Instruments, Gain (Loss) Recognized in Income, Net [Abstract]
Brokergage and Trading Revenue	15	11	30	20
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	96	90	272	360
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	9,796	11,637	20,702	23,275
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	(981)	6,485	(2,864)	6,018
Gain (Loss) on Derivatives, Net	$ (3,057)	$ 3,241	$ (8,742)	$ 2,791