XML 104 R89.htm IDEA: XBRL DOCUMENT
v3.25.4

Other Comprehensive Income (Reclassifications out of Accumulated Other Comprehensive Loss) (Parenthetical) (Details) - Defined Benefit Pension and Postretirement Plans [Member] | 12 Months Ended
--- | ---
Dec. 31, 2025 | Dec. 31, 2024 | Dec. 31, 2023
Condensed Statement Of Income Captions [Line Items] |  |  |
Loss adjustment expenses | 40.00% | 40.00% | 40.00%
Percentage of other operating expenses | 60.00% | 60.00% | 60.00%

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 Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name Regulation S-X (SX) -Number 210 -Section 12 -Subsection 04 -Paragraph a -Publisher SEC Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 942 -SubTopic 235 -Name Accounting Standards Codification -Section S99 -Paragraph 2 -Publisher FASB -URI https://asc.fasb.org/1943274/2147477314/942-235-S99-2 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 235 -SubTopic 10 -Name Accounting Standards Codification -Section S50 -Paragraph 4 -Publisher FASB -URI https://asc.fasb.org/1943274/2147480738/235-10-S50-4 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Topic 235 -SubTopic 10 -Name Accounting Standards Codification -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-04(a)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480678/235-10-S99-3 Reference 5: http://www.xbrl.org/2003/role/recommendedDisclosureRef -Topic 810 -SubTopic 10 -Name Accounting Standards Codification -Section 45 -Paragraph 11 -Publisher FASB -URI https://asc.fasb.org/1943274/2147481231/810-10-45-11 + Details
Name: | srt_CondensedStatementOfIncomeCaptionsLineItems
Namespace Prefix: | srt_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Percentage of amounts reclassified from AOCI for pension and post retirement benefits allocated to loss adjustment expenses. + References No definition available. + Details
Name: | thg_OtherComprehensiveIncomeLossPercentageOfAmountsReclassifiedFromAccumulatedOtherComprehensiveIncomeForPensionAndPostRetirementBenefitsAllocatedToLossAdjustmentExpenses
Namespace Prefix: | thg_
Data Type: | dtr-types:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Percentage of amounts reclassified from AOCI for pension and post retirement benefits allocated to other operating expenses. + References No definition available. + Details
Name: | thg_OtherComprehensiveIncomeLossPercentageOfAmountsReclassifiedFromAccumulatedOtherComprehensiveIncomeForPensionAndPostRetirementBenefitsAllocatedToOtherOperatingExpenses
Namespace Prefix: | thg_
Data Type: | dtr-types:percentItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_StatementEquityComponentsAxis=us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |