Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2023	May 03, 2023
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Mar. 31, 2023
Document Transition Report	false
Entity 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		14,865,525
Entity Central Index Key	0001172052
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2023
Document Fiscal Period Focus	Q1
Amendment Flag	false