Employee Benefit Plans (Net Benefit Cost) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total change in pension obligation, net of tax	$ 13,981	$ 9,720	$ 7,069
BancShares Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	13,582	12,638	12,618
Interest cost	28,376	28,940	28,892
Expected return on assets	(47,867)	(42,074)	(36,643)
Amortization of prior service cost	(79)	(210)	(210)
Amortization of net actuarial loss	13,589	8,855	6,859
Total pension expense	7,759	8,569	11,936
Current year actuarial gain (loss)	(18,407)	(14,834)	(56,268)
Total change in pension obligation, net of tax	4,739	5,769	49,199
Total recognized in net periodic benefit cost and other comprehensive income	12,498	14,338	61,135
Bancorporation Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	2,572	2,548	2,567
Interest cost	6,357	6,653	6,775
Expected return on assets	(12,429)	(11,170)	(11,101)
Amortization of net actuarial loss	313	655	0
Total pension expense	(3,187)	(1,314)	(1,759)
Current year actuarial gain (loss)	(13,605)	1,889	(14,157)
Total change in pension obligation, net of tax	13,292	(2,544)	14,157
Total recognized in net periodic benefit cost and other comprehensive income	$ 10,105	$ (3,858)	$ 12,398