Document And Entity Information	9 Months Ended
Sep. 30, 2011	Nov. 01, 2011
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2011
Document Fiscal Year Focus	2011
Document Fiscal Period Focus	Q3
Trading Symbol	thg
Entity Registrant Name	HANOVER INSURANCE GROUP, INC.
Entity Central Index Key	0000944695
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		44,870,688

If the value is true, then the document as an amendment to previously-filed/accepted document.

Name:	dei_AmendmentFlag
Namespace Prefix:	dei
Data Type:	xbrli:booleanItemType
Balance Type:	na
Period Type:	duration

- Definition

End date of current fiscal year in the format --MM-DD.

+ Details

Name:	dei_CurrentFiscalYearEndDate
Namespace Prefix:	dei
Data Type:	xbrli:gMonthDayItemType
Balance Type:	na
Period Type:	duration

- Definition

This is focus fiscal period of the document report. For a first quarter 2006 quarterly report, which may also provide financial information from prior periods, the first fiscal quarter should be given as the fiscal period focus. Values: FY, Q1, Q2, Q3, Q4, H1, H2, M9, T1, T2, T3, M8, CY.

+ Details

Name:	dei_DocumentFiscalPeriodFocus
Namespace Prefix:	dei
Data Type:	dei:fiscalPeriodItemType
Balance Type:	na
Period Type:	duration

- Definition

This is focus fiscal year of the document report in CCYY format. For a 2006 annual report, which may also provide financial information from prior periods, fiscal 2006 should be given as the fiscal year focus. Example: 2006.

+ Details

Name:	dei_DocumentFiscalYearFocus
Namespace Prefix:	dei
Data Type:	xbrli:gYearItemType
Balance Type:	na
Period Type:	duration

- Definition

The end date of the period reflected on the cover page if a periodic report. For all other reports and registration statements containing historical data, it is the date up through which that historical data is presented. If there is no historical data in the report, use the filing date. The format of the date is CCYY-MM-DD.

+ Details

Name:	dei_DocumentPeriodEndDate
Namespace Prefix:	dei
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

The type of document being provided (such as 10-K, 10-Q, N-1A, etc). The document type is limited to the same value as the supporting SEC submission type, minus any "/A" suffix. The acceptable values are as follows: S-1, S-3, S-4, S-11, F-1, F-3, F-4, F-9, F-10, 6-K, 8-K, 10, 10-K, 10-Q, 20-F, 40-F, N-1A, 485BPOS, 497, NCSR, N-CSR, N-CSRS, N-Q, 10-KT, 10-QT, 20-FT, and Other.

+ Details

Name:	dei_DocumentType
Namespace Prefix:	dei
Data Type:	dei:submissionTypeItemType
Balance Type:	na
Period Type:	duration

- Definition

A unique 10-digit SEC-issued value to identify entities that have filed disclosures with the SEC. It is commonly abbreviated as CIK.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation 12B

-Number 240

-Section 12b

-Subsection 1

+ Details

Name:	dei_EntityCentralIndexKey
Namespace Prefix:	dei
Data Type:	dei:centralIndexKeyItemType
Balance Type:	na
Period Type:	duration

- Definition

Indicate number of shares outstanding of each of registrant's classes of common stock, as of latest practicable date. Where multiple classes exist define each class by adding class of stock items such as Common Class A [Member], Common Class B [Member] onto the Instrument [Domain] of the Entity Listings, Instrument

+ Details

Name:	dei_EntityCommonStockSharesOutstanding
Namespace Prefix:	dei
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Indicate whether the registrant is one of the following: (1) Large Accelerated Filer, (2) Accelerated Filer, (3) Non-accelerated Filer, or (4) Smaller Reporting Company. Definitions of these categories are stated in Rule 12b-2 of the Exchange Act. This information should be based on the registrant's current or most recent filing containing the related disclosure.

+ Details

Name:	dei_EntityFilerCategory
Namespace Prefix:	dei
Data Type:	dei:filerCategoryItemType
Balance Type:	na
Period Type:	duration

- Definition

The exact name of the entity filing the report as specified in its charter, which is required by forms filed with the SEC.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation 12B

-Number 240

-Section 12b

-Subsection 1

+ Details

Name:	dei_EntityRegistrantName
Namespace Prefix:	dei
Data Type:	xbrli:normalizedStringItemType
Balance Type:	na
Period Type:	duration

- Definition

Trading symbol of an instrument as listed on an exchange.

+ Details

Name:	dei_TradingSymbol
Namespace Prefix:	dei
Data Type:	xbrli:normalizedStringItemType
Balance Type:	na
Period Type:	duration

- Definition

Document and Entity Information Abstract

+ Details

Name:	thg_DocumentAndEntityInformationAbstract
Namespace Prefix:	thg
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration