Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2020	Feb. 10, 2021	Jun. 30, 2020
Cover [Abstract]
Document Type	10-K
Document Annual Report	true
Document Transition Report	false
Document Period End Date	Dec. 31, 2020
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 Shares, $0.01 par value per share
Trading Symbol	SAFT
Security Exchange Name	NASDAQ
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
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		14,893,698
Entity Public Float			$ 1,096,353,415
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	FY
Entity Central Index Key	0001172052
Amendment Flag	false
ICFR Auditor Attestation Flag	true