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Employee Benefits - Postretirement Amounts Included in Accumulated Other Comprehensive Income (Detail) - Other Postretirement Plan - USD ($)
$ in Millions
Dec. 31, 2021
Dec. 31, 2020
Accumulated Other Comprehensive Income (Loss) [Line Items]    
Net Actuarial (Gain) Loss $ (5.3) $ (4.9)
Prior Service (Credit) Cost (11.5) (12.4)
Gross Amount in Accumulated Other Comprehensive Income (16.8) (17.3)
Income Tax Effect (4.2) (4.3)
Net Amount in Accumulated Other Comprehensive Income $ (12.6) $ (13.0)