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INVESTMENTS - SCHEDULE OF AMORTIZED COST, GROSS UNREALIZED GAINS AND LOSSES, ESTIMATED FAIR VALUE, AND ALLOWANCE FOR CREDIT LOSSES (Details) - USD ($) $ in Millions | Sep. 30, 2025 | Jun. 30, 2025 | Dec. 31, 2024 | Sep. 30, 2024 | Jun. 30, 2024 | Dec. 31, 2023
--- | --- | --- | --- | --- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Amortized cost | $ 25,219.3 |  |  |  |  |
Allowance for credit losses | (33.0) | $ (39.1) | $ (37.1) | $ (25.9) | $ (39.8) | $ (42.9)
Estimated fair value | 23,405.3 |  | 22,730.1 |  |  |
Corporate securities |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Amortized cost | 14,226.1 |  | 13,672.1 |  |  |
Gross unrealized gains | 158.2 |  | 60.2 |  |  |
Gross unrealized losses | (1,352.0) |  | (1,660.4) |  |  |
Allowance for credit losses | (27.5) | $ (33.1) | (31.1) | $ (24.5) | $ (38.6) | $ (41.7)
Estimated fair value | 13,004.8 |  | 12,040.8 |  |  |
Certificates of deposit |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Amortized cost | 0.0 |  | 470.0 |  |  |
Gross unrealized gains | 0.0 |  | 18.3 |  |  |
Gross unrealized losses | 0.0 |  | 0.0 |  |  |
Allowance for credit losses | 0.0 |  | 0.0 |  |  |
Estimated fair value | 0.0 |  | 488.3 |  |  |
United States Treasury securities and obligations of United States government corporations and agencies |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Amortized cost | 206.2 |  | 214.8 |  |  |
Gross unrealized gains | 0.0 |  | 0.0 |  |  |
Gross unrealized losses | (26.8) |  | (28.6) |  |  |
Allowance for credit losses | 0.0 |  | 0.0 |  |  |
Estimated fair value | 179.4 |  | 186.2 |  |  |
States and political subdivisions |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Amortized cost | 3,346.9 |  | 3,261.9 |  |  |
Gross unrealized gains | 27.3 |  | 12.2 |  |  |
Gross unrealized losses | (375.7) |  | (436.4) |  |  |
Allowance for credit losses | (2.8) |  | (3.4) |  |  |
Estimated fair value | 2,995.7 |  | 2,834.3 |  |  |
Foreign governments |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Amortized cost | 121.9 |  | 107.2 |  |  |
Gross unrealized gains | 1.1 |  | 0.1 |  |  |
Gross unrealized losses | (10.7) |  | (15.3) |  |  |
Allowance for credit losses | (0.6) |  | (0.9) |  |  |
Estimated fair value | 111.7 |  | 91.1 |  |  |
Asset-backed securities |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Amortized cost | 1,547.1 |  | 1,574.6 |  |  |
Gross unrealized gains | 11.9 |  | 8.3 |  |  |
Gross unrealized losses | (43.2) |  | (66.4) |  |  |
Allowance for credit losses | (0.1) |  | (0.1) |  |  |
Estimated fair value | 1,515.7 |  | 1,516.4 |  |  |
Agency residential mortgage-backed securities |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Amortized cost | 896.0 |  | 819.8 |  |  |
Gross unrealized gains | 11.9 |  | 5.3 |  |  |
Gross unrealized losses | (0.5) |  | (5.5) |  |  |
Allowance for credit losses | 0.0 |  | 0.0 |  |  |
Estimated fair value | 907.4 |  | 819.6 |  |  |
Non-agency residential mortgage-backed securities |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Amortized cost | 1,569.7 |  | 1,636.3 |  |  |
Gross unrealized gains | 38.6 |  | 33.6 |  |  |
Gross unrealized losses | (96.9) |  | (130.8) |  |  |
Allowance for credit losses | 0.0 |  | 0.0 |  |  |
Estimated fair value | 1,511.4 |  | 1,539.1 |  |  |
Collateralized loan obligations |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Amortized cost | 1,127.4 |  | 1,015.2 |  |  |
Gross unrealized gains | 3.2 |  | 5.6 |  |  |
Gross unrealized losses | (4.5) |  | (4.0) |  |  |
Allowance for credit losses | 0.0 |  | 0.0 |  |  |
Estimated fair value | 1,126.1 |  | 1,016.8 |  |  |
Commercial mortgage-backed securities |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Amortized cost | 2,178.0 |  | 2,379.1 |  |  |
Gross unrealized gains | 5.9 |  | 3.7 |  |  |
Gross unrealized losses | (128.8) |  | (183.7) |  |  |
Allowance for credit losses | (2.0) |  | (1.6) |  |  |
Estimated fair value | 2,053.1 |  | 2,197.5 |  |  |
Total fixed maturities, available for sale |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Amortized cost | 25,219.3 |  | 25,151.0 |  |  |
Gross unrealized gains | 258.1 |  | 147.3 |  |  |
Gross unrealized losses | (2,039.1) |  | (2,531.1) |  |  |
Allowance for credit losses | (33.0) |  | (37.1) |  |  |
Estimated fair value | $ 23,405.3 |  | $ 22,730.1 |  |  |

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