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Pension and Other Postretirement Benefit Plans - Unrecognized Postretirement Health and Life Costs (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Pension [Member]    
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]    
Total Unrecognized Postretirement Health and Life Cost $ (236.2) $ (250.4)
Postretirement Health and Life [Member]    
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]    
Net Loss [1] 21.1 19.8
Prior Service Credit (4.6) (4.7)
Total Unrecognized Postretirement Health and Life Cost $ 16.5 $ 15.1
[1] Excludes gains, losses and contributions associated with irrevocable grantor trusts.