| Document And Entity Information | Apr. 29, 2021 |
| Document Information [Line Items] | |
| Entity Registrant Name | HANOVER INSURANCE GROUP, INC. |
| Document Type | 8-K |
| Amendment Flag | false |
| Entity Central Index Key | 0000944695 |
| Document Period End Date | Apr. 29, 2021 |
| Entity Emerging Growth Company | false |
| Entity File Number | 1-13754 |
| Entity Incorporation, State or Country Code | DE |
| Entity Tax Identification Number | 04-3263626 |
| Entity Address, Address Line One | 440 Lincoln Street |
| Entity Address, City or Town | Worcester |
| Entity Address, State or Province | MA |
| Entity Address, Postal Zip Code | 01653 |
| City Area Code | 508 |
| Local Phone Number | 855-1000 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Common Stock [Member] | |
| Document Information [Line Items] | |
| Title of each class | Common Stock, $.01 par value |
| Trading Symbol | THG |
| Name of each exchange on which registered | NYSE |
| 7 5/8% Senior Debentures Due 2025 [Member] | |
| Document Information [Line Items] | |
| Title of each class | 7 5/8% Senior Debentures due 2025 |
| Trading Symbol | THG |
| Name of each exchange on which registered | NYSE |
