Employee Benefit Plans (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2007
Defined Benefit Plan Disclosure [Line Items]
Deferred Compensation Cash-based Arrangements, Liability, Current and Noncurrent	$ 28,400	$ 24,500
Defined Benefit Plan, Target Allocation Variance	10.00%
Defined Contribution Plan, Cost Recognized	$ 23,500	22,600	$ 16,400
Employer contributions	$ 50,000	$ 30,000
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%
BancShares Plan
Defined Benefit Plan Disclosure [Line Items]
Actual % of Plan Assets	100.00%	100.00%
Defined Benefit Plan, Accumulated Benefit Obligation	$ 587,327	$ 533,125
Employer contributions	$ 50,000	$ 30,000
Assumed discount rate (percent)	4.68%	4.27%	4.90%
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%
Bancorporation Plan
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Curtailments	$ 0	$ (2,076)	$ 0
Defined Benefit Plan, Accumulated Benefit Obligation	$ 143,676	$ 131,879
Assumed discount rate (percent)	4.68%	4.27%	4.35%
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%
Bancorporation Plan
Defined Benefit Plan Disclosure [Line Items]
Defined Contribution Plan, Cost Recognized			$ 1,100
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%