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Goodwill and Intangible Assets (Narrative) (Detail)
$ in Millions
9 Months Ended
Sep. 30, 2021
USD ($)
segment
Dec. 31, 2020
USD ($)
Dec. 31, 2019
USD ($)
Finite-Lived Intangible Assets [Line Items]      
Number of Reporting Units | segment 7    
Adjustments to purchase price allocations $ (0.1)    
Intangible Assets, Net (Excluding Goodwill) 478.7 $ 519.1 $ 503.7
Intangibles with indefinite useful lives 70.8 $ 70.9  
I&S (acquisition)      
Finite-Lived Intangible Assets [Line Items]      
Adjustments to purchase price allocations $ (0.1)