Employee Benefit Plans (Funded Status Of Plans) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Bancorporation Plan
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Curtailment	$ 0	$ 0	$ (2,076)
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at January 1	156,831	143,241
Service cost	2,548	2,567	3,341
Interest cost	6,653	6,775	6,393
Actuarial (gain) loss	9,168	9,682
Benefits paid	(5,720)	(5,434)
Benefit obligation at December 31	169,480	156,831	143,241
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning balance	152,084	150,893
Actual return on plan assets	22,227	6,625
Fair value of plan assets, ending balance	168,591	152,084	150,893
Funded status	(889)	(4,747)
Defined Benefit Plan, Benefit Obligation, Benefits Paid	5,720	5,434
BancShares Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at January 1	673,227	611,502
Service cost	12,638	12,618	14,083
Interest cost	28,940	28,892	26,975
Actuarial (gain) loss	57,041	40,571
Benefits paid	(21,898)	(20,356)
Benefit obligation at December 31	749,948	673,227	611,502
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning balance	600,616	550,025
Actual return on plan assets	84,281	20,947
Employer contributions	50,000	50,000
Fair value of plan assets, ending balance	712,999	600,616	$ 550,025
Funded status	(36,949)	(72,611)
Defined Benefit Plan, Benefit Obligation, Benefits Paid	$ 21,898	$ 20,356