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, 2019
Dec. 31, 2018
Dec. 31, 2017
Retiree Health and Life Insurance Plans      
Defined Benefit Plan Disclosure [Line Items]      
Service cost $ 0.2 $ 0.4 $ 0.4
Interest cost 2.5 2.6 3.2
Recognized net actuarial loss 3.1 2.5 2.4
Net periodic pension cost (0.4) 0.5 1.2
U.S. Pension Plan | Pension Plan      
Defined Benefit Plan Disclosure [Line Items]      
Service cost 2.7 3.8 5.4
Interest cost 15.0 14.6 15.4
Expected return on plan assets (21.7) (22.2) (22.7)
Amortization of prior service cost (credit) 0.0 0.0 0.4
Recognized net actuarial loss (9.7) (10.7) (10.5)
Net periodic pension cost 5.7 6.9 9.0
Canadian Pension Plan | Pension Plan      
Defined Benefit Plan Disclosure [Line Items]      
Service cost 0.0 0.0  
Interest cost 0.6 0.6 0.6
Expected return on plan assets (0.7) (0.8) (0.9)
Recognized net actuarial loss (0.4) (0.2) (0.2)
Net periodic pension cost $ 0.3 $ 0.0 $ (0.1)