Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2018	Feb. 15, 2019	Jun. 29, 2018
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2018
Document Fiscal Year Focus	2018
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 Current Reporting Status	Yes
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Filer Category	Large Accelerated Filer
Entity Public Float			$ 5,058,103,806
Entity Common Stock, Shares Outstanding		40,573,500
Entity Emerging Growth Company	false
Entity Small Business	false
Entity Shell Company	false