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Employee Benefit Plans - Schedule of Combined Funded Status and Net Periodic Pension Cost and Postretirement Benefit - Postretirement Benefits (Parenthetical) (Detail) - USD ($)
$ in Millions
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2017
Cumulative effect adjustment     $ 1,211.0
ASU 2018-02 [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member]      
Cumulative effect adjustment   $ 30.8  
Other Postretirement Benefits Plan [Member]      
Accumulated other comprehensive loss, tax $ (5.4) (6.1)  
Other Postretirement Benefits Plan [Member] | ASU 2018-02 [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member]      
Cumulative effect adjustment   $ 3.3