Employee Benefit Plans (Narrative) (Details) $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Expected employer contributions in current year	$ 50.0	$ 50.0
BancShares Plan
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Contributions by Employer	0.0	0.0
Bancorporation Plan
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Contributions by Employer	$ 0.0	$ 0.0