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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, 2017
Mar. 31, 2016
Pension Plan, Defined Benefit [Member]    
Service costs $ 0.9 $ 0.9
Interest costs 8.1 8.9
Expected return on plan assets (gains) (10.2) (10.5)
Net periodic benefit cost (credit) (1.2) (0.7)
Postretirement Health and Life Benefits Plans [Member]    
Interest costs 1.8 2.1
Expected return on plan assets (gains) 0.0 (0.1)
Amortization of prior service costs (credits), net (0.1) (0.1)
Net periodic benefit cost (credit) $ 1.7 $ 1.9