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Defined Benefit Plans - Components of Net Periodic Benefit Income for Pension (Detail) - Net Periodic Benefit Income [Member] - Pension Benefits [Member] - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Defined Benefit Plan Disclosure [Line Items]    
Service cost $ 0.1 $ 0.1
Interest cost 6.0 7.1
Expected return on plan assets (8.7) (8.2)
Net periodic benefit income $ (2.6) $ (1.0)