Employee Benefit Plans (Projected Benefit Payments) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
BancShares Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Accumulated Benefit Obligation	$ 659,023	$ 587,327
Defined Benefit Plan, Estimated Future Employer Contributions in Next Fiscal Year	50,000	50,000
2018	26,051
2019	27,514
2020	29,061
2021	30,634
2022	32,074
2023-2027	186,617
Bancorporation Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Accumulated Benefit Obligation	157,613	$ 143,676
2018	6,797
2019	7,099
2020	7,451
2021	7,879
2022	8,364
2023-2027	$ 47,307