Annual report pursuant to Section 13 and 15(d)

Pension and Other Postretirement Benefit Programs - Components of Net Periodic Pension Cost (Details)

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Pension and Other Postretirement Benefit Programs - Components of Net Periodic Pension Cost (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Retiree Health and Life Insurance Plans      
Defined Benefit Plan Disclosure [Line Items]      
Service cost of benefits earned during the period $ 0.4 $ 0.1 $ 0.0
Interest cost on projected benefit obligation 3.3 2.7 0.1
Amortization of prior service cost (credit) 0.0 (0.2) 0.0
Recognized net actuarial loss 2.3 3.2 0.0
Allocated benefit cost (credit) from Shared Plans 0.0 (0.3) 0.7
Net periodic pension cost 1.4 (0.9) 0.8
U.S. Pension Plan | Pension Plan      
Defined Benefit Plan Disclosure [Line Items]      
Service cost of benefits earned during the period 5.4 4.3 0.0
Interest cost on projected benefit obligation 15.4 11.7 0.0
Expected return on plan assets (22.7) (17.4) 0.0
Amortization of prior service cost (credit) 0.4 0.3 0.0
Recognized net actuarial loss (10.5) (7.7) 0.0
Allocated benefit cost (credit) from Shared Plans 0.0 2.2 14.8
Net periodic pension cost 9.0 8.8 14.8
Canadian Pension Plan | Pension Plan      
Defined Benefit Plan Disclosure [Line Items]      
Service cost of benefits earned during the period 0.0 0.0  
Interest cost on projected benefit obligation 0.6 0.5 0.0
Expected return on plan assets (0.9) (0.8) 0.0
Recognized net actuarial loss (0.2) (0.2) 0.0
Allocated benefit cost (credit) from Shared Plans 0.0 0.1 0.3
Net periodic pension cost $ (0.1) $ 0.0 $ 0.3