v3.26.3
Stock-based Compensation (Narrative) (Details) - shares
3 Months Ended 9 Months Ended
Mar. 31, 2026
Mar. 31, 2025
Sep. 30, 2026
Sep. 30, 2025
Time-based, Market-based and Performance-based Restricted Stock Units [Member]        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Vesting period     3 years  
Market-Based Restricted Stock Units [Member]        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Vesting period       3 years
Granted, Shares 0     23,492
Forfeited, Shares 0 14,369    
Decrease as grant in period, percentage     100.00%  
Increase as grant in period, percentage     100.00%  
Market-Based Restricted Stock Units [Member] | Minimum [Member]        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Performance metric, potential range       0.00%
Market-Based Restricted Stock Units [Member] | Maximum [Member]        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Performance metric, potential range       200.00%
Performance-Based and Market-Based Restricted Stock Units [Member]        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Vesting period     3 years  
Granted, Shares     59,663 52,360
Increase or decrease in payout, maximum percentage     20.00%  
Forfeited, Shares     4,273 14,369
Decrease as grant in period, percentage   100.00%    
Performance-Based and Market-Based Restricted Stock Units [Member] | Senior Management [Member]        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Granted, Shares     45,211  
Performance-Based and Market-Based Restricted Stock Units [Member] | Minimum [Member]        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Performance metric, potential range     0.00%  
Performance-Based and Market-Based Restricted Stock Units [Member] | Maximum [Member]        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Performance metric, potential range     240.00%  
Performance-Based Restricted Stock Units [Member]        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Granted, Shares       28,868
Performance-Based Awards [Member]        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Granted, Shares     11,296 5,246
Decrease as grant in period, percentage     100.00%  
Increase as grant in period, percentage     100.00% 100.00%
Performance-Based Awards [Member] | Minimum [Member]        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Performance metric, potential range       0.00%
Performance-Based Awards [Member] | Maximum [Member]        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Performance metric, potential range       200.00%
Hanover Insurance Group 2022 Long-Term Incentive Plan [Member]        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Number of shares available for grant     1,169,466  
Hanover Insurance Group 2023 Employee Stock Purchase Plan [Member]        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Number of shares available for grant     1,201,874