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Pension and Postretirement Health and Life Benefits (Schedule of Net Benefit Costs) (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Pension Plan, Defined Benefit [Member]    
Service costs $ 0.8 $ 0.9
Interest costs 7.2 8.1
Expected return on plan assets (gains) (10.2) (10.2)
Net periodic benefit cost (credit) (2.2) (1.2)
Postretirement Health and Life Benefits Plans [Member]    
Interest costs 1.4 1.8
Amortization of prior service costs (credits), net 0.0 (0.1)
Net periodic benefit cost (credit) $ 1.4 $ 1.7