Employee Benefit Plans (Net Benefit Cost) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total change in pension obligation, net of tax	$ 9,720	$ 7,069	$ 11,586
BancShares Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	12,638	12,618	14,083
Interest cost	28,940	28,892	26,975
Expected return on assets	(42,074)	(36,643)	(33,198)
Amortization of prior service cost	(210)	(210)	(210)
Amortization of net actuarial loss	8,855	6,859	11,376
Total pension expense	8,569	11,936	19,446
Current year actuarial gain (loss)	(14,834)	(56,268)	(927)
Total change in pension obligation, net of tax	5,769	49,199	(10,659)
Total recognized in net periodic benefit cost and other comprehensive income	14,338	61,135	8,787
Bancorporation Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	2,548	2,567	3,341
Interest cost	6,653	6,775	6,393
Expected return on assets	(11,170)	(11,101)	(11,482)
Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Curtailment	0	0	(2,076)
Amortization of net actuarial loss	655	0	0
Total pension expense	(1,314)	(1,759)	(1,748)
Current year actuarial gain (loss)	1,889	(14,157)	(458)
Total change in pension obligation, net of tax	(2,544)	14,157	(1,618)
Total recognized in net periodic benefit cost and other comprehensive income	$ (3,858)	$ 12,398	$ (3,366)