XML 118 R103.htm IDEA: XBRL DOCUMENT
v3.25.4

Liabilities for Outstanding Claims, Losses and Loss Adjustment Expenses (Narrative) (Details) - USD ($) $ in Millions | 12 Months Ended |
--- | --- | ---
Dec. 31, 2025 | Dec. 31, 2024 | Dec. 31, 2023 | Dec. 31, 2022
Liability For Claims And Claims Adjustment Expense [Line Items] |  |  |  |
Catastrophe losses | $ 276.3 | $ 375.9 | $ 690.1 |
(Favorable) unfavorable loss and LAE development | (96.4) | (109.4) | (15.9) |
Loss and loss adjustment expense reserves | $ 7,755.2 | 7,461.2 | 7,308.1 | $ 7,012.6
U.S. Companies [Member] |  |  |  |
Liability For Claims And Claims Adjustment Expense [Line Items] |  |  |  |
Gross incurred claims, percentage | 4.00% |  |  |
Asbestos Issue [Member] |  |  |  |
Liability For Claims And Claims Adjustment Expense [Line Items] |  |  |  |
Loss and loss adjustment expense reserves | $ 27.5 | 55.8 |  |
Personal Lines [Member] |  |  |  |
Liability For Claims And Claims Adjustment Expense [Line Items] |  |  |  |
(Favorable) unfavorable loss and LAE development | (18.2) | 0.1 | 40.9 |
Loss and loss adjustment expense reserves | 2,131.4 | 2,109.0 |  |
Core Commercial [Member] |  |  |  |
Liability For Claims And Claims Adjustment Expense [Line Items] |  |  |  |
(Favorable) unfavorable loss and LAE development | (18.1) | (50.0) | (2.0) |
Loss and loss adjustment expense reserves | 3,861.3 | 3,613.4 |  |
Specialty [Member] |  |  |  |
Liability For Claims And Claims Adjustment Expense [Line Items] |  |  |  |
(Favorable) unfavorable loss and LAE development | (60.8) | (60.9) | (57.1) |
Loss and loss adjustment expense reserves | 1,732.9 | 1,675.9 |  |
Workers' Compensation Line [Member] | Core Commercial [Member] |  |  |  |
Liability For Claims And Claims Adjustment Expense [Line Items] |  |  |  |
(Favorable) unfavorable loss and LAE development |  |  | (2.0) |
Loss and loss adjustment expense reserves | 768.7 | 761.1 |  |
Personal Automobile Lines [Member] | Personal Lines [Member] |  |  |  |
Liability For Claims And Claims Adjustment Expense [Line Items] |  |  |  |
(Favorable) unfavorable loss and LAE development |  |  | $ 40.9 |
Loss and loss adjustment expense reserves | 1,655.4 | 1,635.6 |  |
Specialty Property and Casualty Lines Division, Professional and Executive Lines Division and Marine Division [Member] | Specialty [Member] |  |  |  |
Liability For Claims And Claims Adjustment Expense [Line Items] |  |  |  |
(Favorable) unfavorable loss and LAE development | (60.8) | (60.9) |  |
Commercial Multiple Peril Line [Member] | Core Commercial [Member] |  |  |  |
Liability For Claims And Claims Adjustment Expense [Line Items] |  |  |  |
(Favorable) unfavorable loss and LAE development | (18.1) |  |  |
Loss and loss adjustment expense reserves | $ 1,712.8 | 1,626.9 |  |
Homeowners Line Segment [Member] | Personal Lines [Member] |  |  |  |
Liability For Claims And Claims Adjustment Expense [Line Items] |  |  |  |
(Favorable) unfavorable loss and LAE development |  | (18.2) |  |
Favorable Catastrophe Development [Member] | Core Commercial [Member] |  |  |  |
Liability For Claims And Claims Adjustment Expense [Line Items] |  |  |  |
(Favorable) unfavorable loss and LAE development |  | (32.3) |  |
Other Personal and Homeowners Lines [Member] | Personal Lines [Member] |  |  |  |
Liability For Claims And Claims Adjustment Expense [Line Items] |  |  |  |
(Favorable) unfavorable loss and LAE development |  | $ 0.1 |  |

X
- Definition Shortduration insurance contracts percentage of gross incurred claims. + References No definition available. + Details
Name: | thg_ShortdurationInsuranceContractsPercentageOfGrossIncurredClaims
Namespace Prefix: | thg_
Data Type: | dtr-types:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition The amount of estimated liability as of the balance sheet date for the specific conflagration, earthquake, windstorm, explosion, or similar event. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SAB Topic 5.W.Q2) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479583/944-40-S99-1 + Details
Name: | us-gaap_LiabilityForCatastropheClaimsCarryingAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- 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
Name: | us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpenseLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- 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
Name: | us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersPriorYearClaimsAndClaimsAdjustmentExpense
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Details
Name: | srt_StatementGeographicalAxis=country_US
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_LiabilityForCatastropheClaimsByCatastrophicEventAxis=us-gaap_AsbestosIssueMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_StatementBusinessSegmentsAxis=thg_PersonalLinesSegmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_StatementBusinessSegmentsAxis=thg_CoreCommercialLinesSegmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_StatementBusinessSegmentsAxis=thg_SpecialtyLinesSegmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_SubsegmentsAxis=thg_WorkersCompensationLineMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_SubsegmentsAxis=thg_PersonalAutomobileLinesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_SubsegmentsAxis=thg_SpecialtyPropertyAndCasualtyLinesDivisionProfessionalAndExecutiveLinesDivisionAndMarineDivisionMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_SubsegmentsAxis=thg_CommercialMultiplePerilLineMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_SubsegmentsAxis=thg_HomeownersLineSegmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_SubsegmentsAxis=thg_FavorableCatastropheDevelopmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_SubsegmentsAxis=thg_OtherPersonalAndHomeownersLinesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |