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Business Segments - Earned Premiums by Product Line (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2024 | Dec. 31, 2023 | Dec. 31, 2022
Entity-Wide Information, Revenue from External Customer [Line Items] |  |  |  |
Earned Premiums | [1] | $ 4,215.9 | $ 4,529.4 | $ 5,213.4
Life Insurance |  |  |  |
Entity-Wide Information, Revenue from External Customer [Line Items] |  |  |  |
Earned Premiums |  | (7.2) | (19.1) | 13.3
Operating Segments |  |  |  |
Entity-Wide Information, Revenue from External Customer [Line Items] |  |  |  |
Earned Premiums |  | 3,970.3 | 4,020.1 | 4,617.9
Operating Segments | Specialty Property & Casualty Insurance |  |  |  |
Entity-Wide Information, Revenue from External Customer [Line Items] |  |  |  |
Earned Premiums |  | 3,576.4 | 3,632.5 | 4,046.4
Operating Segments | Specialty Property & Casualty Insurance | Personal Automobile |  |  |  |
Entity-Wide Information, Revenue from External Customer [Line Items] |  |  |  |
Earned Premiums |  | 2,851.4 | 2,977.8 | 3,496.7
Operating Segments | Specialty Property & Casualty Insurance | Commercial Automobile |  |  |  |
Entity-Wide Information, Revenue from External Customer [Line Items] |  |  |  |
Earned Premiums |  | 725.0 | 654.7 | 549.7
Operating Segments | Life Insurance |  |  |  |
Entity-Wide Information, Revenue from External Customer [Line Items] |  |  |  |
Earned Premiums |  | 393.9 | 387.6 | 571.5
Operating Segments | Life Insurance | Life |  |  |  |
Entity-Wide Information, Revenue from External Customer [Line Items] |  |  |  |
Earned Premiums |  | 328.1 | 319.2 | 352.8
Operating Segments | Life Insurance | Accident and Health |  |  |  |
Entity-Wide Information, Revenue from External Customer [Line Items] |  |  |  |
Earned Premiums |  | 22.3 | 23.1 | 168.2
Operating Segments | Life Insurance | Property |  |  |  |
Entity-Wide Information, Revenue from External Customer [Line Items] |  |  |  |
Earned Premiums |  | 43.5 | 45.3 | 50.5
Non-Core Operations |  |  |  |
Entity-Wide Information, Revenue from External Customer [Line Items] |  |  |  |
Earned Premiums |  | $ 245.6 | $ 509.3 | $ 595.5
[1] | 1 Includes a remeasurement loss related to the deferred profit liability within the Life Insurance business of $7.2 million for the year ended December 31, 2024, a remeasurement loss of $19.1 million for the year ended December 31, 2023, and a remeasurement gain of $13.3 million for the year ended December 31, 2022.

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- Definition Amount, after premiums ceded to other entities and premiums assumed by the entity, of premiums earned. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 235 -Name Accounting Standards Codification -Section S99 -Paragraph 2 -Subparagraph (SX 210.12-17(Column E)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147477965/944-235-S99-2 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 605 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (c)(3) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479032/944-605-50-1 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-04(13)(c)) -SubTopic 220 -Topic 942 -Publisher FASB -URI https://asc.fasb.org/1943274/2147478524/942-220-S99-1 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04(1)) -SubTopic 220 -Topic 944 -Publisher FASB -URI https://asc.fasb.org/1943274/2147477250/944-220-S99-1 + Details
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