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Quarterly Financial Data - Additional Information (Detail) - Tradename [Member] - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Dec. 31, 2020
Sep. 30, 2020
Dec. 31, 2019
Sep. 30, 2019
Dec. 31, 2020
Dec. 31, 2019
Quarterly Financial Information [Line Items]            
Pre-tax defined benefit plan actuarial and settlement loss $ 2.6 $ 0.6     $ 3.2  
After-tax defined benefit plan actuarial and settlement gain (loss) $ 2.4 $ 0.4        
Pre-tax defined benefit plan actuarial gain (loss)     $ 32.0 $ 2.1   $ 34.1
After-tax defined benefit plan actuarial gain (loss)     $ 24.2 $ 1.6