Employee Benefit Plans (Net Benefit Cost) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total change in pension obligation, net of tax	$ 11,586	$ 5,358	$ 17,195
BancShares Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	14,083	12,332	16,332
Interest cost	26,975	25,615	23,686
Expected return on assets	(33,198)	(31,269)	(27,733)
Amortization of prior service cost	(210)	(210)	(210)
Amortization of net actuarial loss	11,376	5,148	16,985
Total pension expense	19,446	12,036	29,480
Current year actuarial gain (loss)	(927)	(69,349)	123,557
Total change in pension obligation, net of tax	(10,659)	63,991	(140,752)
Total recognized in net periodic benefit cost and other comprehensive income	8,787	76,027	$ (111,272)
Bancorporation Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	3,341	832
Interest cost	6,393	1,488
Expected return on assets	(11,482)	(2,807)
Defined Benefit Plan, Curtailments	(2,076)	0
Total pension expense	(1,748)	(487)
Current year actuarial gain (loss)	(458)	(9,123)
Total change in pension obligation, net of tax	(1,618)	9,123
Total recognized in net periodic benefit cost and other comprehensive income	$ (3,366)	$ 8,636