Employee Benefit Plans (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2007
Defined Benefit Plan Disclosure [Line Items]
Deferred Compensation Cash-based Arrangements, Liability, Current and Noncurrent	$ 33,400	$ 28,400
Defined Contribution Plan, Cost	$ 25,300	23,500	$ 22,600
Percent of employer match	100.00%			4.50%
Percentage of employee compensation matched	6.00%
Additional contribution if employed at end of year	3.00%
Bancorporation Plan
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Curtailment	$ 0	0	$ (2,076)
Defined Benefit Plan, Accumulated Benefit Obligation	$ 157,613	$ 143,676
Assumed discount rate (percent)	4.30%	4.68%	4.27%
Assumed rate of salary increases	4.00%	4.00%	4.00%
Expected long-term rate of return on plan assets (percent)	7.50%	7.50%	7.50%
BancShares Plan
Defined Benefit Plan Disclosure [Line Items]
Actual % of Plan Assets	100.00%	100.00%
Defined Benefit Plan, Accumulated Benefit Obligation	$ 659,023	$ 587,327
Employer contributions	$ 50,000	$ 50,000
Assumed discount rate (percent)	4.30%	4.68%	4.27%
Assumed rate of salary increases	4.00%	4.00%	4.00%
Expected long-term rate of return on plan assets (percent)	7.50%	7.50%	7.50%
401(k) Matching Range 2
Defined Benefit Plan Disclosure [Line Items]
Percent of employer match				50.00%
Percentage of employee compensation matched				3.00%
401(k) Matching Range 1
Defined Benefit Plan Disclosure [Line Items]
Percent of employer match				100.00%
Percentage of employee compensation matched				3.00%