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Postretirement Benefit Plans - AOCL (Details) - Postretirement Benefit Plans [Member] - USD ($)
$ in Millions
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Amounts recognized in accumulated other comprehensive income:      
Net prior service credit $ (98.2) $ (10.8)  
Accumulated other comprehensive income $ (98.2) $ (10.8) $ (8.1)