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, 2020
Dec. 31, 2019
Dec. 31, 2018
Retiree Health and Life Insurance Plans      
Defined Benefit Plan Disclosure [Line Items]      
Service cost $ 0.0 $ 0.2 $ 0.4
Interest cost 1.9 2.5 2.6
Defined Benefit Plan, Benefit Obligation, Contributions by Plan Participant 1.2 2.2  
Defined Benefit Plan, Benefit Obligation, Increase (Decrease) for Plan Amendment (6.4) (2.6)  
Amortization of prior service cost (credit) (0.2) 0.0 0.0
Recognized net actuarial loss 4.8 3.1 2.5
Net periodic pension cost (3.1) (0.4) 0.5
United States | Pension Plan      
Defined Benefit Plan Disclosure [Line Items]      
Service cost 2.6 2.7 3.8
Interest cost 12.5 15.0 14.6
Expected return on plan assets (21.3) (21.7) (22.2)
Recognized net actuarial loss (10.1) (9.7) (10.7)
Net periodic pension cost 3.9 5.7 6.9
Canadian Pension Plan | Pension Plan      
Defined Benefit Plan Disclosure [Line Items]      
Service cost 0.0 0.0 0.0
Interest cost 0.5 0.6 0.6
Expected return on plan assets (0.5) (0.7) (0.8)
Recognized net actuarial loss (0.3) (0.4) (0.2)
Net periodic pension cost $ 0.3 $ 0.3 $ 0.0