Employee Benefit Plans (Projected Benefit Payments) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
BancShares Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Accumulated Benefit Obligation	$ 626,746	$ 659,023
Defined Benefit Plan, Estimated Future Employer Contributions in Next Fiscal Year	50,000	50,000
2018	27,050
2019	29,137
2020	31,260
2021	33,168
2022	35,031
2023-2027	201,126
Bancorporation Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Accumulated Benefit Obligation	152,344	$ 157,613
2018	7,044
2019	7,473
2020	7,863
2021	8,284
2022	8,685
2023-2027	$ 48,868