v3.26.1
Document and Entity Information - shares
 6 Months Ended
Jun. 30, 2026
Aug. 03, 2026
Document and Entity Information    
Document Type 10-Q  
Document Quarterly Report true  
Document Period End Date Jun. 30, 2026  
Document Transition Report false  
Securities Act File Number 000-50070  
Entity Registrant Name SAFETY INSURANCE GROUP, INC.  
Entity Incorporation, State or Country Code DE  
Entity Tax Identification Number 13-4181699  
Entity Address, Address Line One 20 Custom House Street  
Entity Address, City or Town Boston  
Entity Address, State or Province MA  
Entity Address, Postal Zip Code 02110  
City Area Code 617  
Local Phone Number 951-0600  
Title of 12(b) Security Common Stock, par value $0.01 per share  
Trading Symbol SAFT  
Security Exchange Name NASDAQ  
Entity Current Reporting Status Yes  
Entity Interactive Data Current Yes  
Entity Filer Category Large Accelerated Filer  
Entity Small Business false  
Entity Emerging Growth Company false  
Entity Shell Company false  
Entity Common Stock, Shares Outstanding   14,680,482
Entity Central Index Key 0001172052  
Amendment Flag false  
Current Fiscal Year End Date --12-31  
Document Fiscal Year Focus 2026  
Document Fiscal Period Focus Q2