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Investments Distribution of the Maturity Dates for Fixed Maturity Securities (Details) - USD ($) $ in Millions | Sep. 30, 2025 | Dec. 31, 2024
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Debt Securities, Available-for-sale, Amortized Cost, Net of Allowance for Credit Losses, Fiscal Year Maturity |  |  |
Debt Securities, Available-for-Sale, Amortized Cost, Maturity, Allocated and Single Maturity Date, Year One, Net |  | $ 1,432.4 | $ 1,484.1
Debt Securities, Available-for-Sale, Amortized Cost, Maturity, Allocated and Single Maturity Date, after Year One Through Five, Net |  | 6,736.2 | 7,688.2
Debt Securities, Available-for-Sale, Amortized Cost, Maturity, Allocated and Single Maturity Date, after Year 5 Through 10, Net |  | 7,357.1 | 8,404.6
Debt Securities, Available-for-Sale, Amortized Cost, Maturity, Allocated and Single Maturity Date, after Year 10, Net |  | 18,191.9 | 19,740.8
Available for sale Securities Debt Maturities Amortized Cost Gross Subtotal, Net |  | 33,717.6 | 37,317.7
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Amortized Cost, Net | [1] | 1,128.9 | 949.4
Debt Securities, Available-for-sale, Amortized Cost, Net of Allowance for Credit Losses |  | 34,846.5 | 38,267.1
Available for Sale Securities Unrealized Gain Position Gross Gain |  |  |
Available For Sale Securities Debt Maturities Within One Year Unrealized Gain Position Gross Gain |  | 3.9 | 4.1
Available For Sale Securities Debt Maturities After One Through Five Years Unrealized Gain Position Gross Gain |  | 157.1 | 123.5
Available For Sale Securities Debt Maturities After Five Through Ten Years Unrealized Gain Position Gross Gain |  | 257.5 | 236.4
Available For Sale Securities Debt Maturities After Ten Years Unrealized Gain Position Gross Gain |  | 428.5 | 307.8
Available For Sale Securities Debt Maturities Unrealized Gain Position Gross Gain Gross Subtotal |  | 847.0 | 671.8
Available For Sale Securities Debt Maturities Without Single Maturity Date Unrealized Gain Position Gross Gain | [1] | 10.0 | 5.0
Available for sale Securities, Debt Maturities Unrealized Gain Position Gross Gain |  | 857.0 | 676.8
Available for Sale Securities Unrealized Loss Position Gross Loss |  |  |
Available For Sale Securities Debt Maturities Within One Year Unrealized Loss Position Gross Loss |  | 6.4 | 6.2
Available For Sale Securities Debt Maturities After One Through Five Years Unrealized Loss Position Gross Loss |  | 99.4 | 196.6
Available For Sale Securities Debt Maturities After Five Through Ten Years Unrealized Loss Position Gross Loss |  | 369.7 | 565.5
Available For Sale Securities Debt Maturities After Ten Years Unrealized Loss Position Gross Loss |  | 2,020.5 | 2,508.5
Available For Sale Securities Debt Maturities Unrealized Gain Position Gross Loss Gross Subtotal |  | 2,496.0 | 3,276.8
Available For Sale Securities Debt Maturities Without Single Maturity Date Unrealized Loss Position Gross Loss | [1] | 17.3 | 37.2
Available For Sale Securities Unrealized Loss Position Gross Loss Subtotal |  | 2,513.3 | 3,314.0
Fair Value Maturity Distribution |  |  |
Debt Securities, Available-for-sale, Total |  | 33,190.2 | 35,629.9
Available for Sale Securities Unrealized Gain Position Fair Value |  |  |
Fair Value Maturity Distribution |  |  |
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, within One Year, Fair Value |  | 459.6 | 432.4
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Fair Value |  | 3,523.8 | 2,840.8
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Fair Value |  | 3,651.7 | 3,486.1
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after 10 Years, Fair Value |  | 6,379.9 | 4,965.7
Available for sale Securities Debt Maturities Fair Value Gross Subtotal |  | 14,015.0 | 11,725.0
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Fair Value | [1] | 783.1 | 431.4
Debt Securities, Available-for-sale, Total |  | 14,798.1 | 12,156.4
Available for Sale Securities Unrealized Loss Position Fair Value |  |  |
Fair Value Maturity Distribution |  |  |
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, within One Year, Fair Value |  | 970.3 | 1,049.6
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Fair Value |  | 3,270.1 | 4,774.3
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Fair Value |  | 3,593.2 | 4,589.4
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after 10 Years, Fair Value |  | 10,220.0 | 12,574.4
Available for sale Securities Debt Maturities Fair Value Gross Subtotal |  | 18,053.6 | 22,987.7
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Fair Value | [1] | 338.5 | 485.8
Debt Securities, Available-for-sale, Total |  | $ 18,392.1 | $ 23,473.5
[1] | 1 Includes credit-tranched securities collateralized by loan obligations, auto loans, and other asset types

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