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Retirement Plans (Retirement Savings Plan) (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Retirement Savings Plan [Member]      
Defined Benefit Plan, Plan Assets, Contributions by Employer $ 15.8 $ 15.0 $ 14.1