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Investments - Schedule of Fixed Maturities (Details) - USD ($) $ in Millions | Sep. 30, 2022 | Dec. 31, 2021 | Sep. 30, 2021 | Dec. 31, 2020
--- | --- | --- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Fixed maturities, at amortized cost | $ 7,696.4 |  |  |
Gross unrealized gains | 21.5 |  |  |
Gross unrealized loss | (1,031.3) |  |  |
Fixed maturities, allowance for credit losses | (7.5) | $ (7.5) | $ 4.4 | $ 3.3
Fixed maturities at fair value | 6,679.1 |  |  |
Investments in Fixed Maturities |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Fixed maturities, at amortized cost |  | 7,358.2 |  |
Gross unrealized gains |  | 668.9 |  |
Gross unrealized loss |  | (32.7) |  |
Fixed maturities at fair value |  | 7,986.9 |  |
Debt securities, available-for-sale, unrealized loss position, accumulated loss | 1,031.3 | 32.7 |  |
Investments in Fixed Maturities | Investment-grade |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Debt securities, available-for-sale, unrealized loss position, accumulated loss | 998.2 | 23.7 |  |
U.S. Government and Government Agencies and Authorities |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Fixed maturities, at amortized cost | 612.7 | 610.1 |  |
Gross unrealized gains | 1.3 | 29.2 |  |
Gross unrealized loss | (87.2) | (1.9) |  |
Fixed maturities, allowance for credit losses | 0.0 | 0.0 |  |
Fixed maturities at fair value | 526.8 | 637.4 |  |
Debt securities, available-for-sale, unrealized loss position, accumulated loss | 87.2 | 1.9 |  |
States and Political Subdivisions |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Fixed maturities, at amortized cost | 1,810.0 | 1,752.5 |  |
Gross unrealized gains | 8.9 | 144.6 |  |
Gross unrealized loss | (266.4) | (7.0) |  |
Fixed maturities, allowance for credit losses | (0.4) | 0.0 |  |
Fixed maturities at fair value | 1,552.1 | 1,890.1 |  |
Debt securities, available-for-sale, unrealized loss position, accumulated loss | 266.4 | 7.0 |  |
Foreign Governments |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Fixed maturities, at amortized cost | 4.8 | 6.7 |  |
Gross unrealized gains | 0.0 | 0.0 |  |
Gross unrealized loss | (1.1) | (1.2) |  |
Fixed maturities, allowance for credit losses | 0.0 | 0.0 | 0.0 | 0.3
Fixed maturities at fair value | 3.7 | 5.5 |  |
Debt securities, available-for-sale, unrealized loss position, accumulated loss | 1.1 | 1.2 |  |
Corporate Debt Securities [Member] |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Fixed maturities, at amortized cost | 3,913.9 | 3,929.0 |  |
Gross unrealized gains | 10.5 | 481.4 |  |
Gross unrealized loss | (575.8) | (16.0) |  |
Fixed maturities, allowance for credit losses | (7.1) | (7.5) | 4.4 | $ 3.0
Fixed maturities at fair value | 3,341.5 | 4,386.9 |  |
Debt securities, available-for-sale, unrealized loss position, accumulated loss | 575.8 | 16.0 |  |
Redeemable Preferred Stocks |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Fixed maturities, at amortized cost | 9.0 | 7.0 |  |
Gross unrealized gains | 0.0 | 0.4 |  |
Gross unrealized loss | (1.0) | 0.0 |  |
Fixed maturities, allowance for credit losses | 0.0 | 0.0 |  |
Fixed maturities at fair value | 8.0 | 7.4 |  |
Debt securities, available-for-sale, unrealized loss position, accumulated loss | 1.0 |  | $ 0.0 |
Collateralized Loan Obligations |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Fixed maturities, at amortized cost | 1,006.9 | 756.0 |  |
Gross unrealized gains | 0.0 | 0.9 |  |
Gross unrealized loss | (54.3) | (4.8) |  |
Fixed maturities, allowance for credit losses | 0.0 | 0.0 |  |
Fixed maturities at fair value | 952.6 | 752.1 |  |
Debt securities, available-for-sale, unrealized loss position, accumulated loss | 54.3 | 4.8 |  |
Other Mortgage- and Asset-backed |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Fixed maturities, at amortized cost | 339.1 | 296.9 |  |
Gross unrealized gains | 0.8 | 12.4 |  |
Gross unrealized loss | (45.5) | (1.8) |  |
Fixed maturities, allowance for credit losses | 0.0 | 0.0 |  |
Fixed maturities at fair value | 294.4 | 307.5 |  |
Debt securities, available-for-sale, unrealized loss position, accumulated loss | $ 45.5 | $ 1.8 |  |

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