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Employee Benefit Plans Assets Held in Deferral Plan Irrevocable Grantor Trust Account (Detail) - USD ($)
shares in Millions, $ in Millions
Dec. 31, 2021
Dec. 31, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Assets held in Deferral Plan Irrevocable Grantor Trust account $ 326.2 $ 301.4
Other Investment Funds    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Assets held in Deferral Plan Irrevocable Grantor Trust account 189.1 167.2
Common equities | Progressive common shares    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Assets held in Deferral Plan Irrevocable Grantor Trust account $ 137.1 $ 134.2
Deferred Compensation Arrangement with Individual, Shares Authorized for Issuance 2.4 2.8