Stock-based Compensation (Narrative) (Details) - shares	3 Months Ended
Mar. 31, 2024	Mar. 31, 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	3,803
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	2,372,052
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	1,239,496

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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).

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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.

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