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Consolidated Statements of Comprehensive Income (Unaudited) (Parenthetical) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Statement of Comprehensive Income [Abstract]        
Postretirement adjustments, tax $ 1.9 $ (7.1) $ 2.9 $ (8.1)