Employee Benefit Plans (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2007
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Expected Amortization of Gain (Loss), Next Fiscal Year	$ 25,100
Employer contributions	3,592	$ 50,000
Funded status	(14,334)	(10,441)
Accumulated benefit obligation	$ 904,500	779,100
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	$ 30,800	28,600	$ 25,300
Deferred compensation cash-based arrangements, liability, current and noncurrent	57,000	46,400
BancShares Plan
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	71	50,000
Bancorporation Plan
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	$ 3,500	$ 0
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%