XML 56 R41.htm IDEA: XBRL DOCUMENT
v3.25.4

Pension Plans (Tables) | 12 Months Ended
--- | ---
Dec. 31, 2025
Retirement Plans [Line Items] |
Schedule of Weighted Average Assumptions | Weighted average assumptions used to determine pension benefit obligations are as follows:
DECEMBER 31 |  | 2025 |  |  | 2024 |  |  | 2023 |
Discount rate - qualified plan |  |  | 5.75 | % |  |  | 6.125 | % |  |  | 5.75 | %
Discount rate - non-qualified plan |  |  | 5.625 | % |  |  | 6.125 | % |  |  | 5.875 | %
Cash balance interest crediting rate |  |  | 3.50 | % |  |  | 3.50 | % |  |  | 3.00 | %
The Company utilizes a measurement date of January 1 st to determine its periodic pension costs. Weighted average assumptions used to determine net periodic pension costs for the defined benefit plans are as follows:
YEARS ENDED DECEMBER 31 |  | 2025 |  |  | 2024 |  |  | 2023 |
Qualified plan |  |  |  |  |  |  |  |  |
Discount rate |  |  | 6.125 | % |  |  | 5.75 | % |  |  | 6.00 | %
Expected return on plan assets |  |  | 5.875 | % |  |  | 5.875 | % |  |  | 6.25 | %
Cash balance interest crediting rate |  |  | 3.50 | % |  |  | 3.00 | % |  |  | 3.00 | %
Non-qualified plan |  |  |  |  |  |  |  |  |
Discount rate |  |  | 6.125 | % |  |  | 5.875 | % |  |  | 6.00 | %
Summary of Target Allocations and Invested Asset Allocations | The following table provides its year-end 2025 target allocations and actual invested asset allocations at December 31, 2025 and 2024.
DECEMBER 31 |  | 2025 TARGET LEVELS |  |  | 2025 |  |  | 2024 |
Fixed income securities: |  |  |  |  |  |  |  |  |
Fixed maturities |  |  | 88 | % |  |  | 88 | % |  |  | 88 | %
Money market funds |  |  | 2 | % |  |  | 2 | % |  |  | 2 | %
Total fixed income securities |  |  | 90 | % |  |  | 90 | % |  |  | 90 | %
Equity securities |  |  | 10 | % |  |  | 10 | % |  |  | 10 | %
Total plan assets |  |  | 100 | % |  |  | 100 | % |  |  | 100 | %
Summary of Plan Assets Investment Measured at Fair Value | The following table presents, for each hierarchy level, the qualified defined benefit plan’s investment assets that are measured at fair value at December 31, 2025 and 2024. Refer to Note 4 – “Fair Value” for a description of the different levels in the fair value hierarchy.
DECEMBER 31 |  | 2025 |  |  | 2024 |
(in millions) |  | Total |  |  | Level 1 |  |  | Level 2 |  |  | Level 3 |  |  | Total |  |  | Level 1 |  |  | Level 2 |  |  | Level 3 |
Fixed income securities: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fixed maturities |  | $ | 45.0 |  |  | $ | 36.3 |  |  | $ | — |  |  | $ | 8.7 |  |  | $ | 50.0 |  |  | $ | 39.2 |  |  | $ | — |  |  | $ | 10.8 |
Money market mutual funds |  |  | 6.9 |  |  |  | 6.9 |  |  |  | — |  |  |  | — |  |  |  | 6.4 |  |  |  | 6.4 |  |  |  | — |  |  |  | — |
Total investments at fair value |  | $ | 51.9 |  |  | $ | 43.2 |  |  | $ | — |  |  | $ | 8.7 |  |  | $ | 56.4 |  |  | $ | 45.6 |  |  | $ | — |  |  | $ | 10.8 |
Summary of Assets Measured at Fair Value on a Recurring Basis Using Significant Unobservable Inputs | The table below provides a reconciliation for all assets measured at fair value on a recurring basis using significant unobservable inputs (Level 3).
YEARS ENDED DECEMBER 31 |  | 2025 |  |  | 2024 |
(in millions) |  |  |  |  |  |
Balance at beginning of period |  | $ | 10.8 |  |  | $ | 12.3 |
Less: Assets settled and transferred to Level 1 investments |  |  | (2.4 | ) |  |  | (1.9 | )
Interest earned on plan assets held |  |  | 0.3 |  |  |  | 0.4 |
Balance at end of year |  | $ | 8.7 |  |  | $ | 10.8 |
Schedule of Benefit Obligations, Plan Assets and Funded Status of Plans | The following table reflects the benefit obligations, fair value of plan assets and funded status of the plans at December 31, 2025 and 2024.
DECEMBER 31 |  | Qualified Pension Plan |  |  | Non-Qualified Pension Plan |
(in millions) |  | 2025 |  |  | 2024 |  |  | 2025 |  |  | 2024 |
Change in benefit obligation: |  |  |  |  |  |  |  |  |  |  |  |
Benefit obligation, beginning of period (1) |  | $ | 331.5 |  |  | $ | 348.4 |  |  | $ | 21.3 |  |  | $ | 22.6 |
Interest cost |  |  | 19.2 |  |  |  | 19.1 |  |  |  | 1.3 |  |  |  | 1.3 |
Actuarial (gains) losses |  |  | 7.4 |  |  |  | (3.8 | ) |  |  | 1.1 |  |  |  | 0.2 |
Benefits paid |  |  | (31.7 | ) |  |  | (32.2 | ) |  |  | (2.7 | ) |  |  | (2.8 | )
Benefit obligation, end of year (1) |  |  | 326.4 |  |  |  | 331.5 |  |  |  | 21.0 |  |  |  | 21.3 |
Change in plan assets: |  |  |  |  |  |  |  |  |  |  |  |
Fair value of plan assets, beginning of period |  |  | 340.4 |  |  |  | 363.0 |  |  |  | — |  |  |  | — |
Actual return on plan assets |  |  | 22.0 |  |  |  | 9.6 |  |  |  | — |  |  |  | — |
Contributions |  |  | — |  |  |  | — |  |  |  | 2.7 |  |  |  | 2.8 |
Benefits paid |  |  | (31.7 | ) |  |  | (32.2 | ) |  |  | (2.7 | ) |  |  | (2.8 | )
Fair value of plan assets, end of year |  |  | 330.7 |  |  |  | 340.4 |  |  |  | — |  |  |  | — |
Funded status of the plans |  | $ | 4.3 |  |  | $ | 8.9 |  |  | $ | (21.0 | ) |  | $ | (21.3 | )
(1) The accumulated benefit obligation for these plans is equal to the projected benefit obligation.
Components of Net Periodic Pension Cost | The components of total net periodic pension cost are as follows:
YEARS ENDED DECEMBER 31 |  | 2025 |  |  | 2024 |  |  | 2023 |
(in millions) |  |  |  |  |  |  |  |  |
Interest cost |  | $ | 20.5 |  |  | $ | 20.4 |  |  | $ | 21.9 |
Expected return on plan assets |  |  | (19.0 | ) |  |  | (20.3 | ) |  |  | (22.1 | )
Recognized net actuarial loss |  |  | 6.5 |  |  |  | 6.7 |  |  |  | 7.8 |
Net periodic pension cost |  | $ | 8.0 |  |  | $ | 6.8 |  |  | $ | 7.6 |
Accumulated Other Comprehensive Income (Loss) Related to Postretirement Benefit Plans | The following table reflects the total amounts recognized in accumulated other comprehensive income (loss) relating to the defined benefit pension plans as of December 31, 2025 and 2024.
DECEMBER 31 |  | 2025 |  |  | 2024 |
(in millions) |  |  |  |  |  |
Net actuarial loss |  | $ | 67.2 |  |  | $ | 68.4 |
Schedule of Expected Benefit Payments |
YEARS ENDED DECEMBER 31 |  | 2026 |  |  | 2027 |  |  | 2028 |  |  | 2029 |  |  | 2030 |  |  | 2031-2035 |
(in millions) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Qualified pension plan |  | $ | 35.2 |  |  | $ | 34.3 |  |  | $ | 32.4 |  |  | $ | 31.0 |  |  | $ | 29.1 |  |  | $ | 121.8 |
Non-qualified pension plan |  | $ | 2.6 |  |  | $ | 2.5 |  |  | $ | 2.4 |  |  | $ | 2.3 |  |  | $ | 2.1 |  |  | $ | 8.7 |
Investments Net Asset Value [Member] |
Retirement Plans [Line Items] |
Schedule of Fair Values of Investments | The fair values of these investments are as follows:
DECEMBER 31 |  | 2025 |  |  | 2024 |
Fixed maturities |  | $ | 245.9 |  |  | $ | 250.1 |
Equity securities |  |  | 32.9 |  |  |  | 33.9 |
Total investments carried at NAV |  | $ | 278.8 |  |  | $ | 284.0 |

X
- Definition Retirement Plans [Line Items] + References No definition available. + Details
Name: | thg_RetirementPlansLineItems
Namespace Prefix: | thg_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Schedule of target allocations and invested asset allocations [Table Text Block] + References No definition available. + Details
Name: | thg_ScheduleOfTargetAllocationsAndInvestedAssetAllocationsTableTextBlock
Namespace Prefix: | thg_
Data Type: | dtr-types:textBlockItemType
Balance Type: | na
Period Type: | duration

X
- Definition Tabular disclosure of financial instrument measured at fair value on recurring or nonrecurring basis. Includes, but is not limited to, instrument classified in shareholders' equity. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 Reference 3: http://www.xbrl.org/2003/role/exampleRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 55 -Paragraph 100 -Publisher FASB -URI https://asc.fasb.org/1943274/2147482078/820-10-55-100 Reference 4: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 3 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-3 + Details
Name: | us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTableTextBlock
Namespace Prefix: | us-gaap_
Data Type: | dtr-types:textBlockItemType
Balance Type: | na
Period Type: | duration

X
- Definition Tabular disclosure of the components of accumulated other comprehensive income (loss). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 14A -Publisher FASB -URI https://asc.fasb.org/1943274/2147482790/220-10-45-14A Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 830 -SubTopic 30 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Publisher FASB -URI https://asc.fasb.org/1943274/2147481674/830-30-50-1 Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 830 -SubTopic 30 -Name Accounting Standards Codification -Section 45 -Paragraph 20 -Publisher FASB -URI https://asc.fasb.org/1943274/2147481694/830-30-45-20 + Details
Name: | us-gaap_ScheduleOfAccumulatedOtherComprehensiveIncomeLossTableTextBlock
Namespace Prefix: | us-gaap_
Data Type: | dtr-types:textBlockItemType
Balance Type: | na
Period Type: | duration

X
- Definition Tabular disclosure of the major categories of plan assets of pension plans and/or other employee benefit plans. This information may include, but is not limited to, the target allocation of plan assets, the fair value of each major category of plan assets, and the level within the fair value hierarchy in which the fair value measurements fall. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Topic 715 -SubTopic 20 -Subparagraph (d)(5) -Name Accounting Standards Codification -Paragraph 1 -Section 50 -Publisher FASB -URI https://asc.fasb.org/1943274/2147480506/715-20-50-1 + Details
Name: | us-gaap_ScheduleOfAllocationOfPlanAssetsTableTextBlock
Namespace Prefix: | us-gaap_
Data Type: | dtr-types:textBlockItemType
Balance Type: | na
Period Type: | duration

X
- Definition Tabular disclosure of assumption used to determine benefit obligation and net periodic benefit cost of defined benefit plan. Includes, but is not limited to, discount rate, rate of compensation increase, expected long-term rate of return on plan assets and interest crediting rate. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 5 -Subparagraph (j) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480506/715-20-50-5 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (k) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480506/715-20-50-1 + Details
Name: | us-gaap_ScheduleOfAssumptionsUsedTableTextBlock
Namespace Prefix: | us-gaap_
Data Type: | dtr-types:textBlockItemType
Balance Type: | na
Period Type: | duration

X
- Definition Tabular disclosure of the effect of fair value measurements using significant unobservable inputs (Level 3) on changes in plan assets of pension plans and/or other employee benefit plans for the period. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (d)(iv)(02) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480506/715-20-50-1 + Details
Name: | us-gaap_ScheduleOfEffectOfSignificantUnobservableInputsChangesInPlanAssetsTableTextBlock
Namespace Prefix: | us-gaap_
Data Type: | dtr-types:textBlockItemType
Balance Type: | na
Period Type: | duration

X
- Definition Tabular disclosure of benefits expected to be paid by pension plans and/or other employee benefit plans in each of the next five fiscal years and in the aggregate for the five fiscal years thereafter. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 5 -Subparagraph (e) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480506/715-20-50-5 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Topic 715 -SubTopic 20 -Subparagraph (f) -Name Accounting Standards Codification -Paragraph 1 -Section 50 -Publisher FASB -URI https://asc.fasb.org/1943274/2147480506/715-20-50-1 + Details
Name: | us-gaap_ScheduleOfExpectedBenefitPaymentsTableTextBlock
Namespace Prefix: | us-gaap_
Data Type: | dtr-types:textBlockItemType
Balance Type: | na
Period Type: | duration

X
- Definition Tabular disclosure of the components of net benefit costs for pension plans and/or other employee benefit plans including service cost, interest cost, expected return on plan assets, gain (loss), prior service cost or credit, transition asset or obligation, and gain (loss) recognized due to settlements or curtailments. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 5 -Subparagraph (q) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480506/715-20-50-5 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Topic 715 -SubTopic 20 -Subparagraph (h) -Name Accounting Standards Codification -Paragraph 1 -Section 50 -Publisher FASB -URI https://asc.fasb.org/1943274/2147480506/715-20-50-1 + Details
Name: | us-gaap_ScheduleOfNetBenefitCostsTableTextBlock
Namespace Prefix: | us-gaap_
Data Type: | dtr-types:textBlockItemType
Balance Type: | na
Period Type: | duration

X
- Definition Tabular disclosure of net funded status of pension plans and/or other employee benefit plans. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480506/715-20-50-1 + Details
Name: | us-gaap_ScheduleOfNetFundedStatusTableTextBlock
Namespace Prefix: | us-gaap_
Data Type: | dtr-types:textBlockItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueMeasuredAtNetAssetValuePerShareMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |