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Business Segments - Earned Premiums by Product Line (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2025 | Sep. 30, 2024 | Sep. 30, 2025 | Sep. 30, 2024
Segment Reporting Information [Line Items] |  |  |  |  |
Remeasurement gain | [1] | $ 1,133.3 | $ 1,068.5 | $ 3,352.0 | $ 3,134.1
Life Insurance |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |
Remeasurement gain |  | (0.9) | (0.5) | (1.6) | (1.2)
Operating Segments |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |
Remeasurement gain |  | 1,117.1 | 1,019.6 | 3,290.3 | 2,920.3
Operating Segments | Specialty Property & Casualty Insurance |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |
Remeasurement gain |  | 1,017.3 | 919.0 | 2,990.3 | 2,621.6
Operating Segments | Life Insurance |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |
Remeasurement gain |  | 99.8 | 100.6 | 300.0 | 298.7
Operating Segments | Personal Automobile | Specialty Property & Casualty Insurance |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |
Remeasurement gain |  | 785.1 | 731.3 | 2,328.1 | 2,098.1
Operating Segments | Commercial Automobile | Specialty Property & Casualty Insurance |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |
Remeasurement gain |  | 232.2 | 187.7 | 662.2 | 523.5
Operating Segments | Life | Life Insurance |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |
Remeasurement gain |  | 84.1 | 84.2 | 252.6 | 249.2
Operating Segments | Accident and Health | Life Insurance |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |
Remeasurement gain |  | 5.5 | 5.6 | 16.4 | 16.8
Operating Segments | Property | Life Insurance |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |
Remeasurement gain |  | 10.2 | 10.8 | 31.0 | 32.7
Non-Core Operations |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |
Remeasurement gain |  | $ 16.2 | $ 48.9 | $ 61.7 | $ 213.8
[1] | 1 Includes a remeasurement loss related to the deferred profit liability within the Life insurance business of $0.9 million and $0.5 million for the three months ended September 30, 2025 and 2024, respectively, and a remeasurement loss of $1.6 million and $1.2 million for the nine months ended September 30, 2025 and 2024, respectively.

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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/exampleRef -Topic 944 -SubTopic 605 -Name Accounting Standards Codification -Section 55 -Paragraph 14 -Publisher FASB -URI https://asc.fasb.org/1943274/2147477548/944-605-55-14 Reference 2: http://www.xbrl.org/2003/role/exampleRef -Topic 944 -SubTopic 605 -Name Accounting Standards Codification -Section 55 -Paragraph 11 -Publisher FASB -URI https://asc.fasb.org/1943274/2147477548/944-605-55-11 Reference 3: http://www.xbrl.org/2003/role/exampleRef -Topic 944 -SubTopic 605 -Name Accounting Standards Codification -Section 55 -Paragraph 15 -Publisher FASB -URI https://asc.fasb.org/1943274/2147477548/944-605-55-15 Reference 4: http://www.xbrl.org/2003/role/exampleRef -Topic 944 -SubTopic 605 -Name Accounting Standards Codification -Section 55 -Paragraph 12 -Publisher FASB -URI https://asc.fasb.org/1943274/2147477548/944-605-55-12 Reference 5: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479838/944-20-50-1 Reference 6: http://www.xbrl.org/2003/role/exampleRef -Topic 944 -SubTopic 605 -Name Accounting Standards Codification -Section 55 -Paragraph 11 -Subparagraph (c) -Publisher FASB -URI https://asc.fasb.org/1943274/2147477548/944-605-55-11 Reference 7: 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 8: 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 9: 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 10: 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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