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Document and Entity Information - USD ($)
 12 Months Ended
Dec. 31, 2025
Feb. 18, 2026
Jun. 30, 2025
Cover [Abstract]      
Document Type 10-K    
Amendment Flag false    
Document Period End Date Dec. 31, 2025    
Document Fiscal Year Focus 2025    
Document Fiscal Period Focus FY    
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 Public Float     $ 6,046,846,082
Entity Common Stock, Shares Outstanding   35,149,316  
Entity Current Reporting Status Yes    
Entity Well-known Seasoned Issuer Yes    
Entity Voluntary Filers No    
Document Financial Statement Error Correction [Flag] false    
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 Annual Report true    
Document Transition Report false    
Entity Interactive Data Current Yes    
ICFR Auditor Attestation Flag true    
Auditor Name PricewaterhouseCoopers LLP    
Auditor Firm ID 238    
Auditor Location Boston, Massachusetts    
Auditor Opinion

Opinions on the Financial Statements and Internal Control over Financial Reporting

We have audited the accompanying consolidated balance sheets of The Hanover Insurance Group, Inc. and its subsidiaries (the “Company”) as of December 31, 2025 and 2024, and the related consolidated statements of income, of comprehensive income, of shareholders’ equity and of cash flows for each of the three years in the period ended December 31, 2025, including the related notes and financial statement schedules I, II, III, V, and VI listed in the index appearing under Item 15(a)(2) (collectively referred to as the “consolidated financial statements”). We also have audited the Company’s internal control over financial reporting as of December 31, 2025, based on criteria established in Internal Control - Integrated Framework (2013) issued by the Committee of Sponsoring Organizations of the Treadway Commission (COSO).

In our opinion, the consolidated financial statements referred to above present fairly, in all material respects, the financial position of the Company as of December 31, 2025 and 2024, and the results of its operations and its cash flows for each of the three years in the period ended December 31, 2025 in conformity with accounting principles generally accepted in the United States of America. Also in our opinion, the Company maintained, in all material respects, effective internal control over financial reporting as of December 31, 2025, based on criteria established in Internal Control - Integrated Framework (2013) issued by the COSO.

   
Title of 12(b) Security Common Stock, $.01 par value    
Trading Symbol THG    
Security Exchange Name NYSE    
Documents Incorporated by Reference

Portions of The Hanover Insurance Group, Inc.’s Proxy Statement to be filed pursuant to Regulation 14A relating to the 2026 Annual Meeting of Shareholders to be held May 12, 2026 are incorporated by reference in Part III.