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Pension and Postretirement Health and Life Benefits (Schedule of Net Benefit Costs) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Actuarial (gain) loss from remeasurement, net $ (25.8) $ 1.4
Pension Plan, Defined Benefit [Member]    
Service costs 3.7 3.8
Interest costs 32.2 35.4
Expected return on plan assets (gains) (40.7) (42.0)
Amortization of actuarial (gains) losses, net (10.9) 10.9
Actuarial (gain) loss from remeasurement, net (32.8) (19.1)
Net periodic benefit cost (credit) (15.7) 8.1
Postretirement Health and Life Benefits Plans [Member]    
Service costs 0.0 0.0
Interest costs 7.2 8.2
Expected return on plan assets (gains) 0.0 (0.3)
Amortization of actuarial (gains) losses, net (24.9) (9.5)
Amortization of prior service costs (credits), net 0.1 0.3
Actuarial (gain) loss from remeasurement, net 24.8 9.2
Loss on plan changes resulting from a pending legal settlement 10.0  
Net periodic benefit cost (credit) $ (7.8) $ (1.9)