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Employee Benefits - Postretirement Amounts Included in Accumulated Other Comprehensive Income (Detail) - Other Postretirement Plan - USD ($)
$ in Millions
Dec. 31, 2020
Dec. 31, 2019
Accumulated Other Comprehensive Income (Loss) [Line Items]    
Net Actuarial (Gain) Loss $ (4.9) $ (5.4)
Prior Service Cost (12.4) 0.0
Gross Amount in Accumulated Other Comprehensive Income (17.3) (5.4)
Income Tax Effect (4.3) (1.4)
Net Amount in Accumulated Other Comprehensive Income $ (13.0) $ (4.0)