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Retirement Benefits Plans (Cumulative Amounts Not Yet Recognized in Net Income) (Details) - Postretirement benefit plans - USD ($)
$ in Millions
Dec. 31, 2020
Dec. 31, 2019
Cumulative amounts not yet recognized in net (loss) income:    
Actuarial losses $ 12.1 $ 10.8
Prior service benefits (1.5) (2.0)
Accumulated other comprehensive loss (before related tax benefit) $ 10.6 $ 8.8