Employee Benefit Plans (Funded Status Of Plans) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at January 1	$ 852,975	$ 919,428
Service cost	12,767	16,154	$ 15,186
Interest cost	37,260	34,733	35,593
Actuarial (gain) loss	118,964	(87,752)
Benefits paid	(31,560)	(29,588)
Benefit obligation at December 31	990,406	852,975	919,428
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning balance	842,534	881,590
Actual return on plan assets	161,506	(59,468)
Employer contributions	3,592	50,000
Benefits paid	(31,560)	(29,588)
Fair value of plan assets, ending balance	976,072	842,534	$ 881,590
Funded status	(14,334)	(10,441)
BancShares Plan
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Employer contributions	$ 71	$ 50,000