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Investments - Amortized Cost and Estimated Fair Value of investments (Details) - USD ($) $ in Millions | Dec. 31, 2021 | Dec. 31, 2020 | Dec. 31, 2019
--- | --- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |  |
Amortized cost of fixed maturities | $ 7,358.2 | $ 6,692.7 |
Gross Unrealized Gains | 668.9 | 948.2 |
Gross Unrealized Losses | (32.7) | (31.7) |
Fair Value | 7,986.9 | 7,605.9 |
Debt Securities, Available-for-sale, Allowance for Credit Loss | 7.5 | 3.3 | $ 0.0
United States Government and Government Agencies and Authorities |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Amortized cost of fixed maturities | 610.1 | 536.5 |
Gross Unrealized Gains | 29.2 | 48.9 |
Gross Unrealized Losses | (1.9) | (0.1) |
Fair Value | 637.4 | 585.3 |
Debt Securities, Available-for-sale, Allowance for Credit Loss |  | 0.0 |
States and Political Subdivisions |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Amortized cost of fixed maturities | 1,752.5 | 1,404.3 |
Gross Unrealized Gains | 144.6 | 185.4 |
Gross Unrealized Losses | (7.0) | (0.2) |
Fair Value | 1,890.1 | 1,589.5 |
Debt Securities, Available-for-sale, Allowance for Credit Loss |  | 0.0 |
Foreign Governments |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Amortized cost of fixed maturities | 6.7 | 6.6 |
Gross Unrealized Gains | 0.0 | 0.0 |
Gross Unrealized Losses | (1.2) | (1.1) |
Fair Value | 5.5 | 5.2 |
Debt Securities, Available-for-sale, Allowance for Credit Loss | 0.0 | 0.3 | 0.0
Corporate Bonds and Notes |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Amortized cost of fixed maturities | 3,929.0 | 3,749.5 |
Gross Unrealized Gains | 481.4 | 689.5 |
Gross Unrealized Losses | (16.0) | (10.6) |
Fair Value | 4,386.9 | 4,425.4 |
Debt Securities, Available-for-sale, Allowance for Credit Loss | 7.5 | 3.0 | $ 0.0
Redeemable Preferred Stocks |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Amortized cost of fixed maturities | 7.0 | 7.0 |
Gross Unrealized Gains | 0.4 | 0.5 |
Gross Unrealized Losses | 0.0 | 0.0 |
Fair Value | 7.4 | 7.5 |
Debt Securities, Available-for-sale, Allowance for Credit Loss |  |  |
Collateralized Loan Obligations |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Amortized cost of fixed maturities | 756.0 | 785.1 |
Gross Unrealized Gains | 0.9 | 2.3 |
Gross Unrealized Losses | (4.8) | (19.7) |
Fair Value | 752.1 | 767.7 |
Debt Securities, Available-for-sale, Allowance for Credit Loss |  | 0.0 |
Other Mortgage- and Asset-backed |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Amortized cost of fixed maturities | 296.9 | 203.7 |
Gross Unrealized Gains | 12.4 | 21.6 |
Gross Unrealized Losses | (1.8) | 0.0 |
Fair Value | $ 307.5 | 225.3 |
Debt Securities, Available-for-sale, Allowance for Credit Loss |  | $ 0.0 |

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