Annual report pursuant to Section 13 and 15(d)

Pension and Other Postretirement Benefit Programs - Amounts Recognized in AOCI (Details)

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Pension and Other Postretirement Benefit Programs - Amounts Recognized in AOCI (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Retiree Health and Life Insurance Plans    
Defined Benefit Plan Disclosure [Line Items]    
Net actuarial gain $ 34.8 $ 37.0
U.S. Pension Plan | Pension Plan    
Defined Benefit Plan Disclosure [Line Items]    
Net actuarial gain (127.4) (132.2)
Prior service (cost) 0.0 (0.5)
Accumulated other comprehensive (loss) (127.4) (132.7)
Canadian Pension Plan | Pension Plan    
Defined Benefit Plan Disclosure [Line Items]    
Net actuarial gain (3.8) (2.9)
Prior service (cost) 0.0 0.0
Accumulated other comprehensive (loss) $ (3.8) $ (2.9)