Employee Benefit Plans (Net Benefit Cost) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total change in pension obligation, net of tax	$ 7,069	$ 11,586	$ 5,358
BancShares Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	12,618	14,083	12,332
Interest cost	28,892	26,975	25,615
Expected return on assets	(36,643)	(33,198)	(31,269)
Amortization of prior service cost	(210)	(210)	(210)
Amortization of net actuarial loss	6,859	11,376	5,148
Total pension expense	11,936	19,446	12,036
Current year actuarial gain (loss)	(56,268)	(927)	(69,349)
Total change in pension obligation, net of tax	49,199	(10,659)	63,991
Total recognized in net periodic benefit cost and other comprehensive income	61,135	8,787	76,027
Bancorporation Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	2,567	3,341	832
Interest cost	6,775	6,393	1,488
Expected return on assets	(11,101)	(11,482)	(2,807)
Defined Benefit Plan, Curtailments	0	(2,076)	0
Total pension expense	(1,759)	(1,748)	(487)
Current year actuarial gain (loss)	(14,157)	(458)	(9,123)
Total change in pension obligation, net of tax	14,157	(1,618)	9,123
Total recognized in net periodic benefit cost and other comprehensive income	$ 12,398	$ (3,366)	$ 8,636