Document and Entity Information (USD $)	12 Months Ended
	Dec. 31, 2010	Feb. 18, 2011	Jun. 30, 2010
Document and Entity Information
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2010
Document Fiscal Year Focus	2010
Document Fiscal Period Focus	FY
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		45,247,059
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Well-known Seasoned Issuer	Yes
Entity Public Float			$ 1,927,289,946