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Investments - Financial Assets Measured at Fair Value on Recurring Basis (Detail) - USD ($)
$ in Millions
Dec. 31, 2020
Mar. 31, 2020
Dec. 31, 2019
Sep. 30, 2019
Jun. 30, 2019
Mar. 31, 2019
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]            
Investments $ 5,255.8 $ 4,813.1 $ 5,097.5 $ 5,128.7 $ 4,879.3 $ 4,905.2
Other assets            
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]            
Investments 429.4   486.6      
Fair Value, Recurring            
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]            
Investments 4,928.9   4,707.2      
Fair Value, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)            
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]            
Investments 1,069.6   1,289.1      
Fair Value, Recurring | Significant Other Observable Inputs (Level 2)            
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]            
Investments 3,834.8   3,392.5      
Fair Value, Recurring | Significant Unobservable Inputs (Level 3)            
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]            
Investments 24.5   25.6      
Fair Value, Recurring | Fixed maturities            
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]            
Investments 4,107.1   3,629.9      
Fair Value, Recurring | Fixed maturities | Quoted Prices in Active Markets for Identical Assets (Level 1)            
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]            
Investments 383.5   347.8      
Fair Value, Recurring | Fixed maturities | Significant Other Observable Inputs (Level 2)            
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]            
Investments 3,716.6   3,274.7      
Fair Value, Recurring | Fixed maturities | Significant Unobservable Inputs (Level 3)            
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]            
Investments 7.0   7.4      
Fair Value, Recurring | Fixed maturities | Collateralized loan obligations            
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]            
Investments 289.6   225.8      
Fair Value, Recurring | Fixed maturities | Collateralized loan obligations | Quoted Prices in Active Markets for Identical Assets (Level 1)            
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]            
Investments 0.0   0.0      
Fair Value, Recurring | Fixed maturities | Collateralized loan obligations | Significant Other Observable Inputs (Level 2)            
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]            
Investments 289.6   225.8      
Fair Value, Recurring | Fixed maturities | Collateralized loan obligations | Significant Unobservable Inputs (Level 3)            
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]            
Investments 0.0   0.0      
Fair Value, Recurring | Fixed maturities | U.S. Governments            
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]            
Investments 399.8   353.3      
Fair Value, Recurring | Fixed maturities | U.S. Governments | Quoted Prices in Active Markets for Identical Assets (Level 1)            
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]            
Investments 383.5   347.8      
Fair Value, Recurring | Fixed maturities | U.S. Governments | Significant Other Observable Inputs (Level 2)            
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]            
Investments 16.3   5.5      
Fair Value, Recurring | Fixed maturities | U.S. Governments | Significant Unobservable Inputs (Level 3)            
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]            
Investments 0.0   0.0      
Fair Value, Recurring | Fixed maturities | Foreign Governments            
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]            
Investments 294.8   248.7      
Fair Value, Recurring | Fixed maturities | Foreign Governments | Quoted Prices in Active Markets for Identical Assets (Level 1)            
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]            
Investments 0.0   0.0      
Fair Value, Recurring | Fixed maturities | Foreign Governments | Significant Other Observable Inputs (Level 2)            
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]            
Investments 294.8   248.7      
Fair Value, Recurring | Fixed maturities | Foreign Governments | Significant Unobservable Inputs (Level 3)            
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]            
Investments 0.0   0.0      
Fair Value, Recurring | Fixed maturities | Obligations of states and political subdivisions            
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]            
Investments 170.4   152.6      
Fair Value, Recurring | Fixed maturities | Obligations of states and political subdivisions | Quoted Prices in Active Markets for Identical Assets (Level 1)            
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]            
Investments 0.0   0.0      
Fair Value, Recurring | Fixed maturities | Obligations of states and political subdivisions | Significant Other Observable Inputs (Level 2)            
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]            
Investments 170.4   152.6      
Fair Value, Recurring | Fixed maturities | Obligations of states and political subdivisions | Significant Unobservable Inputs (Level 3)            
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]            
Investments 0.0   0.0      
Fair Value, Recurring | Fixed maturities | Corporate bonds            
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]            
Investments 1,981.8   1,778.4      
Fair Value, Recurring | Fixed maturities | Corporate bonds | Quoted Prices in Active Markets for Identical Assets (Level 1)            
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]            
Investments 0.0   0.0      
Fair Value, Recurring | Fixed maturities | Corporate bonds | Significant Other Observable Inputs (Level 2)            
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]            
Investments 1,974.8   1,771.0      
Fair Value, Recurring | Fixed maturities | Corporate bonds | Significant Unobservable Inputs (Level 3)            
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]            
Investments 7.0   7.4      
Fair Value, Recurring | Fixed maturities | Commercial mortgage-backed securities            
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]            
Investments 339.7   217.0      
Fair Value, Recurring | Fixed maturities | Commercial mortgage-backed securities | Quoted Prices in Active Markets for Identical Assets (Level 1)            
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]            
Investments 0.0   0.0      
Fair Value, Recurring | Fixed maturities | Commercial mortgage-backed securities | Significant Other Observable Inputs (Level 2)            
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]            
Investments 339.7   217.0      
Fair Value, Recurring | Fixed maturities | Commercial mortgage-backed securities | Significant Unobservable Inputs (Level 3)            
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]            
Investments 0.0   0.0      
Fair Value, Recurring | Fixed maturities | Residential mortgage-backed securities            
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]            
Investments 508.2   488.8      
Fair Value, Recurring | Fixed maturities | Residential mortgage-backed securities | Quoted Prices in Active Markets for Identical Assets (Level 1)            
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]            
Investments 0.0   0.0      
Fair Value, Recurring | Fixed maturities | Residential mortgage-backed securities | Significant Other Observable Inputs (Level 2)            
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]            
Investments 508.2   488.8      
Fair Value, Recurring | Fixed maturities | Residential mortgage-backed securities | Significant Unobservable Inputs (Level 3)            
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]            
Investments 0.0   0.0      
Fair Value, Recurring | Fixed maturities | Asset backed securities            
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]            
Investments 122.8   165.3      
Fair Value, Recurring | Fixed maturities | Asset backed securities | Quoted Prices in Active Markets for Identical Assets (Level 1)            
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]            
Investments 0.0   0.0      
Fair Value, Recurring | Fixed maturities | Asset backed securities | Significant Other Observable Inputs (Level 2)            
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]            
Investments 122.8   165.3      
Fair Value, Recurring | Fixed maturities | Asset backed securities | Significant Unobservable Inputs (Level 3)            
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]            
Investments 0.0   0.0      
Fair Value, Recurring | Equity securities            
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]            
Investments 176.7   136.0      
Fair Value, Recurring | Equity securities | Quoted Prices in Active Markets for Identical Assets (Level 1)            
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]            
Investments 159.2   117.8      
Fair Value, Recurring | Equity securities | Significant Other Observable Inputs (Level 2)            
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]            
Investments 0.0   0.0      
Fair Value, Recurring | Equity securities | Significant Unobservable Inputs (Level 3)            
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]            
Investments 17.5   18.2      
Fair Value, Recurring | Other assets            
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]            
Investments 102.5   96.3      
Fair Value, Recurring | Other assets | Quoted Prices in Active Markets for Identical Assets (Level 1)            
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]            
Investments 0.4   0.0      
Fair Value, Recurring | Other assets | Significant Other Observable Inputs (Level 2)            
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]            
Investments 102.1   96.3      
Fair Value, Recurring | Other assets | Significant Unobservable Inputs (Level 3)            
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]            
Investments 0.0   0.0      
Fair Value, Recurring | Other investments, including short-terms            
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]            
Investments 542.6   845.0      
Fair Value, Recurring | Other investments, including short-terms | Quoted Prices in Active Markets for Identical Assets (Level 1)            
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]            
Investments 526.5   823.5      
Fair Value, Recurring | Other investments, including short-terms | Significant Other Observable Inputs (Level 2)            
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]            
Investments 16.1   21.5      
Fair Value, Recurring | Other investments, including short-terms | Significant Unobservable Inputs (Level 3)            
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]            
Investments $ 0.0   $ 0.0