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Consolidated Statements of Comprehensive Income (Unaudited) (Parenthetical) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Statement of Comprehensive Income [Abstract]        
Postretirement adjustments, tax $ 1.4 $ (3.9) $ 4.3 $ (12.0)