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Mortgage Loans, Sorted by Applicable Credit Quality Indicators (Detail) - USD ($) $ in Millions | Mar. 31, 2023 | Dec. 31, 2022 | Mar. 31, 2022 | Dec. 31, 2021
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Mortgage Loans on Real Estate |  |  |  |
Mortgage Loans, Carrying Amount | $ 2,390.0 | $ 2,435.4 |  |
Loan to Value Ratio Below or Equal to 65 Percent |  |  |  |
Mortgage Loans on Real Estate |  |  |  |
Mortgage Loans, Carrying Amount | 1,352.1 | 1,389.6 |  |
Loan To Value Ratio Above 65 To 75 Percent |  |  |  |
Mortgage Loans on Real Estate |  |  |  |
Mortgage Loans, Carrying Amount | 917.8 | 937.2 |  |
Loan To Value Ratio Above 75 To 85 Percent |  |  |  |
Mortgage Loans on Real Estate |  |  |  |
Mortgage Loans, Carrying Amount | 91.5 | 75.0 |  |
Loan To Value Ratio Above 85 Percent |  |  |  |
Mortgage Loans on Real Estate |  |  |  |
Mortgage Loans, Carrying Amount | 28.6 | 33.6 |  |
Originated Five or More Years before Last Fiscal Year |  |  |  |
Mortgage Loans on Real Estate |  |  |  |
Mortgage Loans, Carrying Amount | 1,426.4 | 1,104.2 |  |
Originated Four Years before Last Fiscal Year |  |  |  |
Mortgage Loans on Real Estate |  |  |  |
Mortgage Loans, Carrying Amount | 341.2 | 374.7 |  |
Originated Three Years before Last Fiscal Year |  |  |  |
Mortgage Loans on Real Estate |  |  |  |
Mortgage Loans, Carrying Amount | 168.2 | 343.5 |  |
Originated Two Years before Last Fiscal Year |  |  |  |
Mortgage Loans on Real Estate |  |  |  |
Mortgage Loans, Carrying Amount | 353.3 | 169.3 |  |
Originated in Fiscal Year before Latest Fiscal Year |  |  |  |
Mortgage Loans on Real Estate |  |  |  |
Mortgage Loans, Carrying Amount | 87.8 | 355.6 |  |
Originated in Current Fiscal Year |  |  |  |
Mortgage Loans on Real Estate |  |  |  |
Mortgage Loans, Carrying Amount | 13.1 | 88.1 |  |
Commercial Real Estate |  |  |  |
Mortgage Loans on Real Estate |  |  |  |
Financing Receivable, Originated Five or More Years before Latest Fiscal Year | 1,432.6 | 1,108.5 |  |
Financing Receivable, Originated Four Years before Latest Fiscal Year | 342.6 | 376.5 |  |
Financing Receivable, Originated Three Years before Latest Fiscal Year | 168.8 | 344.8 |  |
Financing Receivable, Originated Two Years before Latest Fiscal Year | 354.4 | 169.9 |  |
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year | 88.4 | 356.4 |  |
Financing Receivable, Originated in Current Fiscal Year | 13.2 | 88.6 |  |
Financing Receivable, before Allowance for Credit Loss | 2,400.0 | 2,444.7 |  |
Financing Receivable, Allowance for Credit Losses, Mortgage Loans | (10.0) | (9.3) |  |
Commercial Real Estate | Loan to Value Ratio Below or Equal to 65 Percent |  |  |  |
Mortgage Loans on Real Estate |  |  |  |
Financing Receivable, Originated Five or More Years before Latest Fiscal Year | 932.6 | 782.6 |  |
Financing Receivable, Originated Four Years before Latest Fiscal Year | 197.0 | 189.0 |  |
Financing Receivable, Originated Three Years before Latest Fiscal Year | 80.8 | 193.9 |  |
Financing Receivable, Originated Two Years before Latest Fiscal Year | 128.0 | 81.3 |  |
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year | 16.7 | 128.9 |  |
Financing Receivable, Originated in Current Fiscal Year | 0.0 | 16.9 |  |
Financing Receivable, before Allowance for Credit Loss | 1,355.1 | 1,392.6 |  |
Financing Receivable, Allowance for Credit Losses, Mortgage Loans | (3.0) | (3.0) | $ (2.4) | $ (2.6)
Commercial Real Estate | Loan To Value Ratio Above 65 To 75 Percent |  |  |  |
Mortgage Loans on Real Estate |  |  |  |
Financing Receivable, Originated Five or More Years before Latest Fiscal Year | 386.6 | 230.2 |  |
Financing Receivable, Originated Four Years before Latest Fiscal Year | 145.6 | 181.6 |  |
Financing Receivable, Originated Three Years before Latest Fiscal Year | 79.6 | 150.9 |  |
Financing Receivable, Originated Two Years before Latest Fiscal Year | 226.4 | 80.1 |  |
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year | 71.7 | 227.5 |  |
Financing Receivable, Originated in Current Fiscal Year | 13.2 | 71.7 |  |
Financing Receivable, before Allowance for Credit Loss | 923.1 | 942.0 |  |
Financing Receivable, Allowance for Credit Losses, Mortgage Loans | (5.3) | (4.7) | (4.5) | (4.7)
Commercial Real Estate | Loan To Value Ratio Above 75 To 85 Percent |  |  |  |
Mortgage Loans on Real Estate |  |  |  |
Financing Receivable, Originated Five or More Years before Latest Fiscal Year | 84.3 | 67.5 |  |
Financing Receivable, Originated Four Years before Latest Fiscal Year | 0.0 | 0.0 |  |
Financing Receivable, Originated Three Years before Latest Fiscal Year | 8.4 | 0.0 |  |
Financing Receivable, Originated Two Years before Latest Fiscal Year | 0.0 | 8.5 |  |
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year | 0.0 | 0.0 |  |
Financing Receivable, Originated in Current Fiscal Year | 0.0 | 0.0 |  |
Financing Receivable, before Allowance for Credit Loss | 92.7 | 76.0 |  |
Financing Receivable, Allowance for Credit Losses, Mortgage Loans | (1.2) | (1.1) | (0.7) | (0.7)
Commercial Real Estate | Loan To Value Ratio Above 85 Percent |  |  |  |
Mortgage Loans on Real Estate |  |  |  |
Financing Receivable, Originated Five or More Years before Latest Fiscal Year | 29.1 | 28.2 |  |
Financing Receivable, Originated Four Years before Latest Fiscal Year | 0.0 | 5.9 |  |
Financing Receivable, Originated Three Years before Latest Fiscal Year | 0.0 | 0.0 |  |
Financing Receivable, Originated Two Years before Latest Fiscal Year | 0.0 | 0.0 |  |
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year | 0.0 | 0.0 |  |
Financing Receivable, Originated in Current Fiscal Year | 0.0 | 0.0 |  |
Financing Receivable, before Allowance for Credit Loss | 29.1 | 34.1 |  |
Financing Receivable, Allowance for Credit Losses, Mortgage Loans | (0.5) | (0.5) | $ (0.3) | $ (0.3)
Commercial Real Estate | Originated Five or More Years before Last Fiscal Year |  |  |  |
Mortgage Loans on Real Estate |  |  |  |
Financing Receivable, Allowance for Credit Losses, Mortgage Loans | (6.2) | (4.3) |  |
Commercial Real Estate | Originated Four Years before Last Fiscal Year |  |  |  |
Mortgage Loans on Real Estate |  |  |  |
Financing Receivable, Allowance for Credit Losses, Mortgage Loans | (1.4) | (1.8) |  |
Commercial Real Estate | Originated Three Years before Last Fiscal Year |  |  |  |
Mortgage Loans on Real Estate |  |  |  |
Financing Receivable, Allowance for Credit Losses, Mortgage Loans | (0.6) | (1.3) |  |
Commercial Real Estate | Originated Two Years before Last Fiscal Year |  |  |  |
Mortgage Loans on Real Estate |  |  |  |
Financing Receivable, Allowance for Credit Losses, Mortgage Loans | (1.1) | (0.6) |  |
Commercial Real Estate | Originated in Fiscal Year before Latest Fiscal Year |  |  |  |
Mortgage Loans on Real Estate |  |  |  |
Financing Receivable, Allowance for Credit Losses, Mortgage Loans | (0.6) | (0.8) |  |
Commercial Real Estate | Originated in Current Fiscal Year |  |  |  |
Mortgage Loans on Real Estate |  |  |  |
Financing Receivable, Allowance for Credit Losses, Mortgage Loans | (0.1) | (0.5) |  |
AA Credit Rating |  |  |  |
Mortgage Loans on Real Estate |  |  |  |
Mortgage Loans, Carrying Amount | 80.2 | 92.3 |  |
AA Credit Rating | Commercial Real Estate |  |  |  |
Mortgage Loans on Real Estate |  |  |  |
Financing Receivable, Originated Five or More Years before Latest Fiscal Year | 69.3 | 53.8 |  |
Financing Receivable, Originated Four Years before Latest Fiscal Year | 11.0 | 27.5 |  |
Financing Receivable, Originated Three Years before Latest Fiscal Year | 0.0 | 11.1 |  |
Financing Receivable, Originated Two Years before Latest Fiscal Year | 0.0 | 0.0 |  |
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year | 0.0 | 0.0 |  |
Financing Receivable, Originated in Current Fiscal Year | 0.0 | 0.0 |  |
Financing Receivable, before Allowance for Credit Loss | 80.3 | 92.4 |  |
A Credit Rating |  |  |  |
Mortgage Loans on Real Estate |  |  |  |
Mortgage Loans, Carrying Amount | 803.7 | 843.9 |  |
A Credit Rating | Commercial Real Estate |  |  |  |
Mortgage Loans on Real Estate |  |  |  |
Financing Receivable, Originated Five or More Years before Latest Fiscal Year | 569.6 | 485.0 |  |
Financing Receivable, Originated Four Years before Latest Fiscal Year | 96.0 | 123.3 |  |
Financing Receivable, Originated Three Years before Latest Fiscal Year | 35.7 | 96.8 |  |
Financing Receivable, Originated Two Years before Latest Fiscal Year | 80.3 | 35.9 |  |
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year | 23.8 | 80.6 |  |
Financing Receivable, Originated in Current Fiscal Year | 0.0 | 24.0 |  |
Financing Receivable, before Allowance for Credit Loss | 805.4 | 845.6 |  |
BBB Credit Rating |  |  |  |
Mortgage Loans on Real Estate |  |  |  |
Mortgage Loans, Carrying Amount | 1,465.2 | 1,458.0 |  |
BBB Credit Rating | Commercial Real Estate |  |  |  |
Mortgage Loans on Real Estate |  |  |  |
Financing Receivable, Originated Five or More Years before Latest Fiscal Year | 752.4 | 534.0 |  |
Financing Receivable, Originated Four Years before Latest Fiscal Year | 235.6 | 219.8 |  |
Financing Receivable, Originated Three Years before Latest Fiscal Year | 133.1 | 236.9 |  |
Financing Receivable, Originated Two Years before Latest Fiscal Year | 274.1 | 134.0 |  |
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year | 64.6 | 275.8 |  |
Financing Receivable, Originated in Current Fiscal Year | 13.2 | 64.6 |  |
Financing Receivable, before Allowance for Credit Loss | 1,473.0 | 1,465.1 |  |
BB Credit Rating |  |  |  |
Mortgage Loans on Real Estate |  |  |  |
Mortgage Loans, Carrying Amount | 40.9 | 41.2 |  |
BB Credit Rating | Commercial Real Estate |  |  |  |
Mortgage Loans on Real Estate |  |  |  |
Financing Receivable, Originated Five or More Years before Latest Fiscal Year | 41.3 | 35.7 |  |
Financing Receivable, Originated Four Years before Latest Fiscal Year | 0.0 | 5.9 |  |
Financing Receivable, Originated Three Years before Latest Fiscal Year | 0.0 | 0.0 |  |
Financing Receivable, Originated Two Years before Latest Fiscal Year | 0.0 | 0.0 |  |
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year | 0.0 | 0.0 |  |
Financing Receivable, Originated in Current Fiscal Year | 0.0 | 0.0 |  |
Financing Receivable, before Allowance for Credit Loss | $ 41.3 | $ 41.6 |  |

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