Derivatives, Derivatives Instruments Gain (Loss) in Income Statement (Details) (Not Designated as Hedging Instrument [Member], USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Trading Activity, Gains and Losses, Net [Line Items]
Brokerage and Trading Revenue	$ 37,800	$ 41,952	$ 38,132
Gain (Loss) on Derivatives, Net	2,776	(4,367)	(301)
To-be-announced residential mortgage-backed securities [Member]
Trading Activity, Gains and Losses, Net [Line Items]
Brokerage and Trading Revenue	27,007	29,614	25,509
Gain (Loss) on Derivatives, Net	0	0	0
Interest rate swaps [Member]
Trading Activity, Gains and Losses, Net [Line Items]
Brokerage and Trading Revenue	2,494	2,991	3,458
Gain (Loss) on Derivatives, Net	0	0	0
Energy contracts [Member]
Trading Activity, Gains and Losses, Net [Line Items]
Brokerage and Trading Revenue	6,572	8,303	8,171
Gain (Loss) on Derivatives, Net	0	0	0
Agricultural contracts [Member]
Trading Activity, Gains and Losses, Net [Line Items]
Brokerage and Trading Revenue	146	357	382
Gain (Loss) on Derivatives, Net	0	0	0
Foreign exchange contracts [Member]
Trading Activity, Gains and Losses, Net [Line Items]
Brokerage and Trading Revenue	1,581	687	612
Gain (Loss) on Derivatives, Net	0	0	0
Equity option contracts [Member]
Trading Activity, Gains and Losses, Net [Line Items]
Brokerage and Trading Revenue	0	0	0
Gain (Loss) on Derivatives, Net	0	0	0
Customer risk management programs [Member]
Trading Activity, Gains and Losses, Net [Line Items]
Brokerage and Trading Revenue	37,800	41,952	38,132
Gain (Loss) on Derivatives, Net	0	0	0
Interest rate risk management programs [Member]
Trading Activity, Gains and Losses, Net [Line Items]
Brokerage and Trading Revenue	0	0	0
Gain (Loss) on Derivatives, Net	$ 2,776	$ (4,367)	$ (301)