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Mortgage Loans, Sorted by Applicable Credit Quality Indicators (Detail) - USD ($) $ in Millions | Sep. 30, 2025 | Jun. 30, 2025 | Dec. 31, 2024 | Sep. 30, 2024 | Jun. 30, 2024 | Dec. 31, 2023
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Mortgage Loans on Real Estate |  |  |  |  |  |  |
Financing Receivable, Allowance for Credit Losses, Mortgage Loans |  | $ 15.6 |  | $ 16.1 |  |  |
Mortgage Loans, Carrying Amount | [1],[2] | 2,129.8 |  | 2,224.5 |  |  |
Loan to Value Ratio Below or Equal to 65 Percent |  |  |  |  |  |  |
Mortgage Loans on Real Estate |  |  |  |  |  |  |
Mortgage Loans, Carrying Amount | [2] | 1,648.1 |  | 1,639.6 |  |  |
Loan To Value Ratio Above 65 To 75 Percent |  |  |  |  |  |  |
Mortgage Loans on Real Estate |  |  |  |  |  |  |
Mortgage Loans, Carrying Amount | [2] | 252.7 |  | 367.6 |  |  |
Loan To Value Ratio Above 75 To 85 Percent |  |  |  |  |  |  |
Mortgage Loans on Real Estate |  |  |  |  |  |  |
Mortgage Loans, Carrying Amount | [2] | 132.6 |  | 152.3 |  |  |
Loan To Value Ratio Above 85 Percent |  |  |  |  |  |  |
Mortgage Loans on Real Estate |  |  |  |  |  |  |
Mortgage Loans, Carrying Amount | [2] | 96.4 |  | 65.0 |  |  |
Originated Five or More Years before Last Fiscal Year |  |  |  |  |  |  |
Mortgage Loans on Real Estate |  |  |  |  |  |  |
Mortgage Loans, Carrying Amount | [1] | 1,517.3 |  | 1,541.0 |  |  |
Originated Four Years before Last Fiscal Year |  |  |  |  |  |  |
Mortgage Loans on Real Estate |  |  |  |  |  |  |
Mortgage Loans, Carrying Amount | [1] | 322.9 |  | 154.2 |  |  |
Originated Three Years before Last Fiscal Year |  |  |  |  |  |  |
Mortgage Loans on Real Estate |  |  |  |  |  |  |
Mortgage Loans, Carrying Amount | [1] | 85.7 |  | 329.6 |  |  |
Originated Two Years before Last Fiscal Year |  |  |  |  |  |  |
Mortgage Loans on Real Estate |  |  |  |  |  |  |
Mortgage Loans, Carrying Amount | [1] | 65.9 |  | 87.3 |  |  |
Originated in Fiscal Year before Latest Fiscal Year |  |  |  |  |  |  |
Mortgage Loans on Real Estate |  |  |  |  |  |  |
Mortgage Loans, Carrying Amount | [1] | 45.7 |  | 66.4 |  |  |
Originated in Current Fiscal Year |  |  |  |  |  |  |
Mortgage Loans on Real Estate |  |  |  |  |  |  |
Mortgage Loans, Carrying Amount | [1] | 92.3 |  | 46.0 |  |  |
Commercial Real Estate |  |  |  |  |  |  |
Mortgage Loans on Real Estate |  |  |  |  |  |  |
Financing Receivable, Originated Five or More Years before Latest Fiscal Year | [1] | 1,530.7 |  | 1,554.7 |  |  |
Financing Receivable, Originated Four Years before Latest Fiscal Year | [1] | 323.8 |  | 154.7 |  |  |
Financing Receivable, Originated Three Years before Latest Fiscal Year | [1] | 86.0 |  | 330.6 |  |  |
Financing Receivable, Originated Two Years before Latest Fiscal Year | [1] | 66.3 |  | 87.6 |  |  |
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year | [1] | 45.9 |  | 66.8 |  |  |
Financing Receivable, Originated in Current Fiscal Year | [1] | 92.7 |  | 46.2 |  |  |
Financing Receivable, before Allowance for Credit Loss | [1] | 2,145.4 |  | 2,240.6 |  |  |
Financing Receivable, Allowance for Credit Losses, Mortgage Loans | [1] | 15.6 |  | 16.1 |  |  |
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 | [1] | 1,267.1 |  | 1,229.6 |  |  |
Financing Receivable, Originated Four Years before Latest Fiscal Year | [1] | 240.6 |  | 112.9 |  |  |
Financing Receivable, Originated Three Years before Latest Fiscal Year | [1] | 39.7 |  | 210.0 |  |  |
Financing Receivable, Originated Two Years before Latest Fiscal Year | [1] | 38.5 |  | 40.8 |  |  |
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year | [1] | 11.6 |  | 38.7 |  |  |
Financing Receivable, Originated in Current Fiscal Year | [1] | 54.7 |  | 11.7 |  |  |
Financing Receivable, before Allowance for Credit Loss | [1] | 1,652.2 |  | 1,643.7 |  |  |
Financing Receivable, Allowance for Credit Losses, Mortgage Loans | [1] | 4.1 | $ 3.8 | 4.2 | $ 4.0 | $ 3.7 | $ 3.8
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 | [1] | 117.0 |  | 154.1 |  |  |
Financing Receivable, Originated Four Years before Latest Fiscal Year | [1] | 10.8 |  | 33.7 |  |  |
Financing Receivable, Originated Three Years before Latest Fiscal Year | [1] | 46.3 |  | 72.1 |  |  |
Financing Receivable, Originated Two Years before Latest Fiscal Year | [1] | 27.8 |  | 46.8 |  |  |
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year | [1] | 34.3 |  | 28.1 |  |  |
Financing Receivable, Originated in Current Fiscal Year | [1] | 17.6 |  | 34.5 |  |  |
Financing Receivable, before Allowance for Credit Loss | [1] | 253.8 |  | 369.3 |  |  |
Financing Receivable, Allowance for Credit Losses, Mortgage Loans | [1] | 1.1 | 1.5 | 1.7 | 2.2 | 3.3 | 3.8
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 | [1] | 98.7 |  | 126.4 |  |  |
Financing Receivable, Originated Four Years before Latest Fiscal Year | [1] | 35.1 |  | 8.1 |  |  |
Financing Receivable, Originated Three Years before Latest Fiscal Year | [1] | 0.0 |  | 20.1 |  |  |
Financing Receivable, Originated Two Years before Latest Fiscal Year | [1] | 0.0 |  | 0.0 |  |  |
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year | [1] | 0.0 |  | 0.0 |  |  |
Financing Receivable, Originated in Current Fiscal Year | [1] | 0.0 |  | 0.0 |  |  |
Financing Receivable, before Allowance for Credit Loss | [1] | 133.8 |  | 154.6 |  |  |
Financing Receivable, Allowance for Credit Losses, Mortgage Loans | [1] | 1.2 | 1.1 | 2.2 | 2.5 | 1.7 | 1.2
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 | [1] | 47.9 |  | 44.6 |  |  |
Financing Receivable, Originated Four Years before Latest Fiscal Year | [1] | 37.3 |  | 0.0 |  |  |
Financing Receivable, Originated Three Years before Latest Fiscal Year | [1] | 0.0 |  | 28.4 |  |  |
Financing Receivable, Originated Two Years before Latest Fiscal Year | [1] | 0.0 |  | 0.0 |  |  |
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year | [1] | 0.0 |  | 0.0 |  |  |
Financing Receivable, Originated in Current Fiscal Year | [1] | 20.4 |  | 0.0 |  |  |
Financing Receivable, before Allowance for Credit Loss | [1] | 105.6 |  | 73.0 |  |  |
Financing Receivable, Allowance for Credit Losses, Mortgage Loans | [1] | 9.2 | $ 9.2 | 8.0 | $ 7.8 | $ 3.4 | $ 1.4
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 | [1] | 13.4 |  | 13.7 |  |  |
Commercial Real Estate | Originated Four Years before Last Fiscal Year |  |  |  |  |  |  |
Mortgage Loans on Real Estate |  |  |  |  |  |  |
Financing Receivable, Allowance for Credit Losses, Mortgage Loans | [1] | 0.9 |  | 0.5 |  |  |
Commercial Real Estate | Originated Three Years before Last Fiscal Year |  |  |  |  |  |  |
Mortgage Loans on Real Estate |  |  |  |  |  |  |
Financing Receivable, Allowance for Credit Losses, Mortgage Loans | [1] | 0.3 |  | 1.0 |  |  |
Commercial Real Estate | Originated Two Years before Last Fiscal Year |  |  |  |  |  |  |
Mortgage Loans on Real Estate |  |  |  |  |  |  |
Financing Receivable, Allowance for Credit Losses, Mortgage Loans | [1] | 0.4 |  | 0.3 |  |  |
Commercial Real Estate | Originated in Fiscal Year before Latest Fiscal Year |  |  |  |  |  |  |
Mortgage Loans on Real Estate |  |  |  |  |  |  |
Financing Receivable, Allowance for Credit Losses, Mortgage Loans | [1] | 0.2 |  | 0.4 |  |  |
Commercial Real Estate | Originated in Current Fiscal Year |  |  |  |  |  |  |
Mortgage Loans on Real Estate |  |  |  |  |  |  |
Financing Receivable, Allowance for Credit Losses, Mortgage Loans | [1] | 0.4 |  | 0.2 |  |  |
AA Credit Rating |  |  |  |  |  |  |
Mortgage Loans on Real Estate |  |  |  |  |  |  |
Mortgage Loans, Carrying Amount |  | 105.0 |  | 117.8 |  |  |
AA Credit Rating | Commercial Real Estate |  |  |  |  |  |  |
Mortgage Loans on Real Estate |  |  |  |  |  |  |
Financing Receivable, Originated Five or More Years before Latest Fiscal Year |  | 98.9 |  | 111.5 |  |  |
Financing Receivable, Originated Four Years before Latest Fiscal Year |  | 6.1 |  | 0.0 |  |  |
Financing Receivable, Originated Three Years before Latest Fiscal Year |  | 0.0 |  | 6.4 |  |  |
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 |  | 105.0 |  | 117.9 |  |  |
A Credit Rating |  |  |  |  |  |  |
Mortgage Loans on Real Estate |  |  |  |  |  |  |
Mortgage Loans, Carrying Amount |  | 1,058.7 |  | 1,099.1 |  |  |
A Credit Rating | Commercial Real Estate |  |  |  |  |  |  |
Mortgage Loans on Real Estate |  |  |  |  |  |  |
Financing Receivable, Originated Five or More Years before Latest Fiscal Year |  | 806.5 |  | 780.5 |  |  |
Financing Receivable, Originated Four Years before Latest Fiscal Year |  | 165.6 |  | 99.6 |  |  |
Financing Receivable, Originated Three Years before Latest Fiscal Year |  | 23.8 |  | 169.1 |  |  |
Financing Receivable, Originated Two Years before Latest Fiscal Year |  | 9.5 |  | 24.6 |  |  |
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year |  | 6.4 |  | 9.5 |  |  |
Financing Receivable, Originated in Current Fiscal Year |  | 49.0 |  | 18.0 |  |  |
Financing Receivable, before Allowance for Credit Loss |  | 1,060.8 |  | 1,101.3 |  |  |
BBB Credit Rating |  |  |  |  |  |  |
Mortgage Loans on Real Estate |  |  |  |  |  |  |
Mortgage Loans, Carrying Amount |  | 840.6 |  | 915.5 |  |  |
BBB Credit Rating | Commercial Real Estate |  |  |  |  |  |  |
Mortgage Loans on Real Estate |  |  |  |  |  |  |
Financing Receivable, Originated Five or More Years before Latest Fiscal Year |  | 520.2 |  | 561.7 |  |  |
Financing Receivable, Originated Four Years before Latest Fiscal Year |  | 142.8 |  | 55.1 |  |  |
Financing Receivable, Originated Three Years before Latest Fiscal Year |  | 62.2 |  | 155.1 |  |  |
Financing Receivable, Originated Two Years before Latest Fiscal Year |  | 56.8 |  | 63.0 |  |  |
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year |  | 39.5 |  | 57.3 |  |  |
Financing Receivable, Originated in Current Fiscal Year |  | 23.3 |  | 28.2 |  |  |
Financing Receivable, before Allowance for Credit Loss |  | 844.8 |  | 920.4 |  |  |
BB Credit Rating |  |  |  |  |  |  |
Mortgage Loans on Real Estate |  |  |  |  |  |  |
Mortgage Loans, Carrying Amount |  | 110.5 |  | 85.0 |  |  |
BB Credit Rating | Commercial Real Estate |  |  |  |  |  |  |
Mortgage Loans on Real Estate |  |  |  |  |  |  |
Financing Receivable, Originated Five or More Years before Latest Fiscal Year |  | 82.5 |  | 86.8 |  |  |
Financing Receivable, Originated Four Years before Latest Fiscal Year |  | 9.3 |  | 0.0 |  |  |
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 |  | 20.4 |  | 0.0 |  |  |
Financing Receivable, before Allowance for Credit Loss |  | 112.2 |  | 86.8 |  |  |
B Credit Rating |  |  |  |  |  |  |
Mortgage Loans on Real Estate |  |  |  |  |  |  |
Mortgage Loans, Carrying Amount |  | 15.0 |  | 7.1 |  |  |
B Credit Rating | Commercial Real Estate |  |  |  |  |  |  |
Mortgage Loans on Real Estate |  |  |  |  |  |  |
Financing Receivable, Originated Five or More Years before Latest Fiscal Year |  | 22.6 |  | 14.2 |  |  |
Financing Receivable, Originated Four Years before Latest Fiscal Year |  | 0.0 |  | 0.0 |  |  |
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 |  | $ 22.6 |  | $ 14.2 |  |  |
[1] | 1 Loan-to-Value Ratio utilizes the most recent internal valuation of the property
[2] | Loan-to-Value Ratio utilizes the most recent internal valuation of the property

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