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Liabilities for Outstanding Claims, Losses and Loss Adjustment Expenses (Schedule of Carried Reserves) (Details) - USD ($) $ in Millions | Dec. 31, 2025 | Dec. 31, 2024 | Dec. 31, 2023 | Dec. 31, 2022
--- | --- | --- | --- | ---
Liability For Claims And Claims Adjustment Expense [Line Items] |  |  |  |
Gross | $ 7,755.2 | $ 7,461.2 | $ 7,308.1 | $ 7,012.6
Ceded | (1,850.3) | (1,829.8) | (1,795.0) | (1,748.6)
Net | 5,904.9 | 5,631.4 | $ 5,513.1 | $ 5,264.0
Core Commercial Lines Segment [Member] |  |  |  |
Liability For Claims And Claims Adjustment Expense [Line Items] |  |  |  |
Gross | 3,861.3 | 3,613.4 |  |
Ceded | (716.4) | (624.2) |  |
Net | 3,144.9 | 2,989.2 |  |
Specialty Lines Segment [Member] |  |  |  |
Liability For Claims And Claims Adjustment Expense [Line Items] |  |  |  |
Gross | 1,732.9 | 1,675.9 |  |
Ceded | (322.9) | (334.6) |  |
Net | 1,410.0 | 1,341.3 |  |
Other Segment [Member] |  |  |  |
Liability For Claims And Claims Adjustment Expense [Line Items] |  |  |  |
Gross | 29.6 | 62.9 |  |
Ceded | (17.3) | (50.3) |  |
Net | 12.3 | 12.6 |  |
Personal Lines [Member] |  |  |  |
Liability For Claims And Claims Adjustment Expense [Line Items] |  |  |  |
Gross | 2,131.4 | 2,109.0 |  |
Ceded | (793.7) | (820.7) |  |
Net | 1,337.7 | 1,288.3 |  |
Commercial Multiple Peril Line [Member] | Core Commercial Lines Segment [Member] |  |  |  |
Liability For Claims And Claims Adjustment Expense [Line Items] |  |  |  |
Gross | 1,712.8 | 1,626.9 |  |
Ceded | (247.0) | (202.0) |  |
Net | 1,465.8 | 1,424.9 |  |
Workers' Compensation Line [Member] | Core Commercial Lines Segment [Member] |  |  |  |
Liability For Claims And Claims Adjustment Expense [Line Items] |  |  |  |
Gross | 768.7 | 761.1 |  |
Ceded | (129.3) | (131.6) |  |
Net | 639.4 | 629.5 |  |
Commercial Automobile Line [Member] | Core Commercial Lines Segment [Member] |  |  |  |
Liability For Claims And Claims Adjustment Expense [Line Items] |  |  |  |
Gross | 581.7 | 513.5 |  |
Ceded | (27.1) | (22.3) |  |
Net | 554.6 | 491.2 |  |
General Liability and Umbrella - Occurrence [Member] | Core Commercial Lines Segment [Member] |  |  |  |
Liability For Claims And Claims Adjustment Expense [Line Items] |  |  |  |
Gross | 697.2 | 620.7 |  |
Ceded | (293.7) | (258.2) |  |
Net | 403.5 | 362.5 |  |
Other Core Commercial [Member] | Core Commercial Lines Segment [Member] |  |  |  |
Liability For Claims And Claims Adjustment Expense [Line Items] |  |  |  |
Gross | 100.9 | 91.2 |  |
Ceded | (19.3) | (10.1) |  |
Net | 81.6 | 81.1 |  |
Specialty Property & Casualty [Member] | Specialty Lines Segment [Member] |  |  |  |
Liability For Claims And Claims Adjustment Expense [Line Items] |  |  |  |
Gross | 845.3 | 837.9 |  |
Ceded | (249.7) | (257.3) |  |
Net | 595.6 | 580.6 |  |
General Liability - Claims Made [Member] | Specialty Lines Segment [Member] |  |  |  |
Liability For Claims And Claims Adjustment Expense [Line Items] |  |  |  |
Gross | 472.9 | 454.2 |  |
Ceded | (30.6) | (30.6) |  |
Net | 442.3 | 423.6 |  |
Other Specialty [Member] | Specialty Lines Segment [Member] |  |  |  |
Liability For Claims And Claims Adjustment Expense [Line Items] |  |  |  |
Gross | 414.7 | 383.8 |  |
Ceded | (42.6) | (46.7) |  |
Net | 372.1 | 337.1 |  |
Personal Automobile Lines [Member] | Personal Lines [Member] |  |  |  |
Liability For Claims And Claims Adjustment Expense [Line Items] |  |  |  |
Gross | 1,655.4 | 1,635.6 |  |
Ceded | (787.6) | (817.4) |  |
Net | 867.8 | 818.2 |  |
Homeowners [Member] | Personal Lines [Member] |  |  |  |
Liability For Claims And Claims Adjustment Expense [Line Items] |  |  |  |
Gross | 300.8 | 323.6 |  |
Ceded | (3.9) | (1.5) |  |
Net | 296.9 | 322.1 |  |
Other Personal [Member] | Personal Lines [Member] |  |  |  |
Liability For Claims And Claims Adjustment Expense [Line Items] |  |  |  |
Gross | 175.2 | 149.8 |  |
Ceded | (2.2) | (1.8) |  |
Net | $ 173.0 | $ 148.0 |  |

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- Definition The amount needed to reflect the estimated ultimate cost of settling claims relating to insured events that have occurred on or before the balance sheet date, whether or not reported to the insurer at that date. + References Reference 1: 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 2: http://www.xbrl.org/2003/role/exampleRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 55 -Paragraph 7 -Publisher FASB -URI https://asc.fasb.org/1943274/2147480046/944-40-55-7 Reference 3: http://www.xbrl.org/2003/role/exampleRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 55 -Paragraph 9E -Publisher FASB -URI https://asc.fasb.org/1943274/2147480046/944-40-55-9E Reference 4: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 50 -Paragraph 3 -Subparagraph (cc) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480081/944-40-50-3 Reference 5: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 944 -SubTopic 210 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(a)(13)(a)(1)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147478777/944-210-S99-1 + Details
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- Definition Liability as of the balance sheet date for amounts representing estimated cost of settling unpaid claims under the terms of the underlying insurance policies, less estimated reinsurance recoveries on such claims. This includes an estimate for claims which have been incurred but not reported. Claim adjustment expenses represent the costs estimated to be incurred in the settlement of unpaid claims. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 944 -SubTopic 210 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(a)(13)(a)(1)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147478777/944-210-S99-1 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 50 -Paragraph 3 -Subparagraph (cc) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480081/944-40-50-3 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 50 -Paragraph 3 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480081/944-40-50-3 + Details
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- Definition Amount, after valuation allowance, recoverable under reinsurance contracts for losses reported to the ceding insurer but not yet paid and amounts expected for incurred losses and settlement expenses, which have not yet been reported to the ceding insurer. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 944 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 5 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479838/944-20-50-5 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 944 -SubTopic 310 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Publisher FASB -URI https://asc.fasb.org/1943274/2147477363/944-310-50-2 Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 944 -SubTopic 310 -Name Accounting Standards Codification -Section 45 -Paragraph 6 -Publisher FASB -URI https://asc.fasb.org/1943274/2147477409/944-310-45-6 Reference 4: http://www.xbrl.org/2003/role/exampleRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 55 -Paragraph 7 -Publisher FASB -URI https://asc.fasb.org/1943274/2147480046/944-40-55-7 Reference 5: http://www.xbrl.org/2003/role/exampleRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 55 -Paragraph 9E -Publisher FASB -URI https://asc.fasb.org/1943274/2147480046/944-40-55-9E Reference 6: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 50 -Paragraph 3 -Subparagraph (cc) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480081/944-40-50-3 Reference 7: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 50 -Paragraph 3 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480081/944-40-50-3 Reference 8: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 50 -Paragraph 4C -Publisher FASB -URI https://asc.fasb.org/1943274/2147480081/944-40-50-4C + Details
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