Employee Benefit Plans (Funded Status Of Plans) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at January 1	$ 990,406	$ 852,975
Service cost	14,279	12,767	$ 16,154
Interest cost	34,197	37,260	34,733
Actuarial (gain) loss	72,080	118,964
Benefits paid	(33,309)	(31,560)
Benefit obligation at December 31	1,077,653	990,406	852,975
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning balance	976,072	842,534
Actual return on plan assets	192,792	161,506
Employer contributions	100,000	3,592
Benefits paid	(33,309)	(31,560)
Fair value of plan assets, ending balance	1,235,555	976,072	$ 842,534
Funded status	157,902	(14,334)
BancShares Plan
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Employer contributions	$ 80,000	$ 71