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Liabilities for Outstanding Claims, Losses and Loss Adjustment Expenses (Narrative) (Details) - USD ($) $ in Millions | 9 Months Ended
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Sep. 30, 2024 | Sep. 30, 2023
Liability For Claims And Claims Adjustment Expense [Line Items] |  |
(Favorable) unfavorable loss and LAE development | $ (78.8) | $ (6.8)
Personal Automobile Lines [Member] |  |
Liability For Claims And Claims Adjustment Expense [Line Items] |  |
(Favorable) unfavorable loss and LAE development |  | 16.4
Other Personal Lines [Member] |  |
Liability For Claims And Claims Adjustment Expense [Line Items] |  |
(Favorable) unfavorable loss and LAE development | (10.3) |
Surety Lines [Member] |  |
Liability For Claims And Claims Adjustment Expense [Line Items] |  |
(Favorable) unfavorable loss and LAE development | 14.1 | 9.2
Specialty [Member] |  |
Liability For Claims And Claims Adjustment Expense [Line Items] |  |
(Favorable) unfavorable loss and LAE development | (34.8) | 43.1
Specialty [Member] | Professional and Executive Lines [Member] |  |
Liability For Claims And Claims Adjustment Expense [Line Items] |  |
(Favorable) unfavorable loss and LAE development |  | (28.6)
Specialty [Member] | Favorable Catastrophe Development [Member] |  |
Liability For Claims And Claims Adjustment Expense [Line Items] |  |
(Favorable) unfavorable loss and LAE development | 24.8 |
Core Commercial [Member] |  |
Liability For Claims And Claims Adjustment Expense [Line Items] |  |
(Favorable) unfavorable loss and LAE development | (39.7) | 1.2
Core Commercial [Member] | Commercial Multiple Peril Line [Member] |  |
Liability For Claims And Claims Adjustment Expense [Line Items] |  |
(Favorable) unfavorable loss and LAE development |  | (13.5)
Core Commercial [Member] | Workers' Compensation Line [Member] |  |
Liability For Claims And Claims Adjustment Expense [Line Items] |  |
(Favorable) unfavorable loss and LAE development |  | 17.7
Core Commercial [Member] | Certain Liability Lines [Member] |  |
Liability For Claims And Claims Adjustment Expense [Line Items] |  |
(Favorable) unfavorable loss and LAE development | (10.7) |
Core Commercial [Member] | Commercial Multiple Peril, Commercial Automobile, Workers' Compensation And Other Commercial Lines [Member] |  |
Liability For Claims And Claims Adjustment Expense [Line Items] |  |
(Favorable) unfavorable loss and LAE development | (25.6) |
Personal Lines [Member] |  |
Liability For Claims And Claims Adjustment Expense [Line Items] |  |
(Favorable) unfavorable loss and LAE development | (4.3) | 35.1
Personal Lines [Member] | Personal Automobile Lines [Member] |  |
Liability For Claims And Claims Adjustment Expense [Line Items] |  |
(Favorable) unfavorable loss and LAE development | $ (13.4) | $ 11.4

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- Definition Amount, after effects of reinsurance, of expense (reversal of expense) for claims incurred in prior reporting periods and related claims settlement costs. + References Reference 1: 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 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 50 -Paragraph 3 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480081/944-40-50-3 + Details
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