Employee Benefit Plans (Net Benefit Cost) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total change in pension obligation, net of tax	$ 5,358	$ 17,195	$ 11,236
BancShares Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	12,332	16,332	14,241
Interest cost	25,615	23,686	23,711
Expected return on assets	(31,269)	(27,733)	(28,478)
Amortization of prior service cost	(210)	(210)	(210)
Amortization of net actuarial loss	5,148	16,985	11,026
Total pension expense	12,036	29,480	20,710
Current year actuarial gain (loss)	(69,349)	123,557	(44,315)
Total change in pension obligation, net of tax	63,991	(140,752)	33,079
Total recognized in net periodic benefit cost and other comprehensive income	76,027	(111,272)	53,789
Bancorporation Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	832
Interest cost	1,488
Expected return on assets	(2,807)
Amortization of prior service cost	0
Amortization of net actuarial loss	0
Total pension expense	(487)
Current year actuarial gain (loss)	(9,123)
Total change in pension obligation, net of tax	9,123
Total recognized in net periodic benefit cost and other comprehensive income	$ 8,636