Document and Entity Information	3 Months Ended
	Mar. 31, 2013	Apr. 26, 2013
Document Information [Line Items]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2013
Document Fiscal Year Focus	2013
Document Fiscal Period Focus	Q1
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		44,012,080