Employee Benefit Plans (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2007
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	$ 100,000	$ 3,592
Funded status	157,902	(14,334)
Accumulated benefit obligation	$ 985,000	904,500
Percent of employer match (percent)	100.00%			4.50%
Percentage of employee compensation matched	6.00%
Additional contribution if employed at end of year	3.00%
Defined contribution plan cost	$ 35,600	30,800	$ 28,600
Deferred compensation cash-based arrangements, liability, current and noncurrent	68,200	57,000
BancShares Plan
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	80,000	71
Bancorporation Plan
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	$ 20,000	$ 3,500
401(k) Matching Range 1
Defined Benefit Plan Disclosure [Line Items]
Percent of employer match (percent)				100.00%
Percentage of employee compensation matched				3.00%
401(k) Matching Range 2
Defined Benefit Plan Disclosure [Line Items]
Percent of employer match (percent)				50.00%
Percentage of employee compensation matched				3.00%