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Employee Benefit Plans (Narrative) (Details) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2018 USD ($) | Sep. 30, 2018 USD ($)
BancShares Plan |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Expected employer contributions in current year | $ 50.0 | $ 50.0
Defined Benefit Plan, Plan Assets, Contributions by Employer | 50.0 | 50.0
Bancorporation Plan |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Defined Benefit Plan, Plan Assets, Contributions by Employer | $ 0.0 | $ 0.0

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