v3.26.1
Document and Entity Information - shares
 6 Months Ended
Jun. 30, 2026
Jul. 27, 2026
Cover [Abstract]    
Document Type 10-Q  
Amendment Flag false  
Document Period End Date Jun. 30, 2026  
Document Fiscal Year Focus 2026  
Document Fiscal Period Focus Q2  
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 Emerging Growth Company false  
Entity Small Business false  
Entity Common Stock, Shares Outstanding   34,820,221
Entity Current Reporting Status Yes  
Entity Shell 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  
Document Quarterly Report true  
Document Transition Report false  
Entity Interactive Data Current Yes  
Title of 12(b) Security Common Stock, $.01 par value  
Trading Symbol THG  
Security Exchange Name NYSE