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Stock-based Compensation (Narrative) (Details) - shares | 9 Months Ended
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Sep. 30, 2024 | Sep. 30, 2023
Time-based, Market-based and Performance-based Restricted Stock Units [Member] |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |
Vesting period | 3 years |
Market-Based Restricted Stock Units [Member] |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |
Vesting period | 3 years |
Granted, Shares | 25,414 | 21,789
Forfeited, Shares | 6,339 | 2,836
Decrease as grant in period, percentage | 100.00% |
Performance-Based and Market-Based Restricted Stock Units [Member] |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |
Granted, Shares | 53,336 | 50,115
Forfeited, Shares | 6,339 | 3,993
Performance-Based and Market-Based Restricted Stock Units [Member] | Minimum [Member] | Senior Management [Member] |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |
Performance metric, potential range | 0.00% |
Performance-Based and Market-Based Restricted Stock Units [Member] | Maximum [Member] | Senior Management [Member] |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |
Performance metric, potential range | 150.00% |
Performance-Based Restricted Stock Units [Member] |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |
Granted, Shares | 27,922 | 28,326
Performance-Based Restricted Stock Units [Member] | Minimum [Member] |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |
Performance metric, potential range | 0.00% |
Performance-Based Restricted Stock Units [Member] | Maximum [Member] |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |
Performance metric, potential range | 150.00% |
Performance-Based Awards [Member] |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |
Granted, Shares | 2,615 | 5,961
Increase as grant in period, percentage | 100.00% |
Hanover Insurance Group 2022 Long-Term Incentive Plan [Member] |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |
Number of shares available for grant | 2,356,776 |
Hanover Insurance Group 2023 Employee Stock Purchase Plan [Member] |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |
Number of shares available for grant | 1,229,903 |

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- Definition Period over which grantee's right to exercise award under share-based payment arrangement is no longer contingent on satisfaction of service or performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, combination of market, performance or service condition. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 718 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (a)(1) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480429/718-10-50-2 + Details
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- Definition The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 718 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (c)(2)(iii)(03) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480429/718-10-50-2 + Details
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- Definition The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 718 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (c)(2)(iii)(01) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480429/718-10-50-2 + Details
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- Definition Line items represent financial concepts included in a table. 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