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Investments - Components of Net Investment Income (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2023 | Sep. 30, 2022 | Sep. 30, 2023 | Sep. 30, 2022
Net Investment Income [Line Items] |  |  |  |
Gross investment income | $ 510.2 | $ 333.6 | $ 1,384.3 | $ 868.2
Investment expenses | (7.2) | (5.8) | (18.8) | (17.4)
Net Investment Income | 503.0 | 327.8 | 1,365.5 | 850.8
Available-for-sale Securities |  |  |  |
Net Investment Income [Line Items] |  |  |  |
Gross investment income | 487.6 | 304.8 | 1,311.1 | 766.2
Total equity securities |  |  |  |
Net Investment Income [Line Items] |  |  |  |
Gross investment income | 22.6 | 28.8 | 73.2 | 102.0
Total equity securities | Nonredeemable preferred stocks |  |  |  |
Net Investment Income [Line Items] |  |  |  |
Gross investment income | 11.9 | 17.3 | 40.3 | 53.6
Total equity securities | Common equities |  |  |  |
Net Investment Income [Line Items] |  |  |  |
Gross investment income | 10.7 | 11.5 | 32.9 | 48.4
Total fixed maturities |  |  |  |
Net Investment Income [Line Items] |  |  |  |
Gross investment income | 466.0 | 280.0 | 1,240.0 | 736.6
Total fixed maturities | U.S. government obligations |  |  |  |
Net Investment Income [Line Items] |  |  |  |
Gross investment income | 237.0 | 86.1 | 591.7 | 205.4
Total fixed maturities | State and local government obligations |  |  |  |
Net Investment Income [Line Items] |  |  |  |
Gross investment income | 12.3 | 9.8 | 34.5 | 29.3
Total fixed maturities | Foreign government obligations |  |  |  |
Net Investment Income [Line Items] |  |  |  |
Gross investment income | 0.1 | 0.1 | 0.2 | 0.2
Total fixed maturities | Corporate debt securities |  |  |  |
Net Investment Income [Line Items] |  |  |  |
Gross investment income | 94.8 | 71.8 | 261.9 | 215.4
Total fixed maturities | Residential mortgage-backed securities |  |  |  |
Net Investment Income [Line Items] |  |  |  |
Gross investment income | 7.6 | 8.8 | 22.6 | 23.2
Total fixed maturities | Commercial mortgage-backed securities |  |  |  |
Net Investment Income [Line Items] |  |  |  |
Gross investment income | 48.7 | 48.3 | 147.9 | 138.1
Total fixed maturities | Other asset-backed securities |  |  |  |
Net Investment Income [Line Items] |  |  |  |
Gross investment income | 62.9 | 52.1 | 173.0 | 116.5
Total fixed maturities | Redeemable preferred stocks |  |  |  |
Net Investment Income [Line Items] |  |  |  |
Gross investment income | 2.6 | 3.0 | 8.2 | 8.5
Short-term investments |  |  |  |
Net Investment Income [Line Items] |  |  |  |
Gross investment income | $ 21.6 | $ 24.8 | $ 71.1 | $ 29.6

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