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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 | Mar. 31, 2025 | Dec. 31, 2024 | Mar. 31, 2024 | Dec. 31, 2023
--- | --- | --- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Amortized cost | $ 25,471.1 |  |  |
Allowance for credit losses | (38.9) | $ (37.1) | $ (39.0) | $ (42.9)
Estimated fair value | 23,283.0 | 22,840.5 |  |
Corporate securities |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Amortized cost | 13,942.0 | 13,785.3 |  |
Gross unrealized gains | 80.1 | 60.4 |  |
Gross unrealized losses | (1,553.8) | (1,663.3) |  |
Allowance for credit losses | (33.1) | (31.1) | $ (37.7) | $ (41.7)
Estimated fair value | 12,435.2 | 12,151.3 |  |
Certificates of deposit |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Amortized cost | 470.0 | 470.0 |  |
Gross unrealized gains | 0.7 | 18.3 |  |
Gross unrealized losses | 0.0 | 0.0 |  |
Allowance for credit losses | 0.0 | 0.0 |  |
Estimated fair value | 470.7 | 488.3 |  |
United States Treasury securities and obligations of United States government corporations and agencies |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Amortized cost | 216.3 | 214.8 |  |
Gross unrealized gains | 0.0 | 0.0 |  |
Gross unrealized losses | (22.9) | (28.6) |  |
Allowance for credit losses | 0.0 | 0.0 |  |
Estimated fair value | 193.4 | 186.2 |  |
States and political subdivisions |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Amortized cost | 3,297.6 | 3,261.9 |  |
Gross unrealized gains | 23.1 | 12.2 |  |
Gross unrealized losses | (383.4) | (436.4) |  |
Allowance for credit losses | (3.1) | (3.4) |  |
Estimated fair value | 2,934.2 | 2,834.3 |  |
Foreign governments |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Amortized cost | 109.0 | 107.3 |  |
Gross unrealized gains | 0.3 | 0.1 |  |
Gross unrealized losses | (14.4) | (15.3) |  |
Allowance for credit losses | (1.0) | (0.9) |  |
Estimated fair value | 93.9 | 91.2 |  |
Asset-backed securities |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Amortized cost | 1,625.7 | 1,574.6 |  |
Gross unrealized gains | 9.3 | 8.3 |  |
Gross unrealized losses | (58.4) | (66.5) |  |
Allowance for credit losses | (0.1) | (0.1) |  |
Estimated fair value | 1,576.5 | 1,516.3 |  |
Agency residential mortgage-backed securities |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Amortized cost | 812.1 | 819.8 |  |
Gross unrealized gains | 8.1 | 5.3 |  |
Gross unrealized losses | (1.6) | (5.5) |  |
Allowance for credit losses | 0.0 | 0.0 |  |
Estimated fair value | 818.6 | 819.6 |  |
Non-agency residential mortgage-backed securities |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Amortized cost | 1,665.0 | 1,636.3 |  |
Gross unrealized gains | 37.0 | 33.5 |  |
Gross unrealized losses | (111.8) | (130.8) |  |
Allowance for credit losses | 0.0 | 0.0 |  |
Estimated fair value | 1,590.2 | 1,539.0 |  |
Collateralized loan obligations |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Amortized cost | 1,009.2 | 1,015.2 |  |
Gross unrealized gains | 2.2 | 5.6 |  |
Gross unrealized losses | (6.5) | (4.0) |  |
Allowance for credit losses | 0.0 | 0.0 |  |
Estimated fair value | 1,004.9 | 1,016.8 |  |
Commercial mortgage-backed securities |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Amortized cost | 2,324.2 | 2,379.1 |  |
Gross unrealized gains | 3.6 | 3.7 |  |
Gross unrealized losses | (160.8) | (183.7) |  |
Allowance for credit losses | (1.6) | (1.6) |  |
Estimated fair value | 2,165.4 | 2,197.5 |  |
Total fixed maturities, available for sale |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Amortized cost | 25,471.1 | 25,264.3 |  |
Gross unrealized gains | 164.4 | 147.4 |  |
Gross unrealized losses | (2,313.6) | (2,534.1) |  |
Allowance for credit losses | (38.9) | (37.1) |  |
Estimated fair value | $ 23,283.0 | $ 22,840.5 |  |

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