Document and Entity Information	9 Months Ended
Sep. 30, 2011	Nov. 03, 2011
Document Information Line Items
Document Type	10-Q
Document Period End Date	Sep. 30, 2011
Amendment Flag	false
Document Fiscal Period Focus	Q3
Document Fiscal Year Focus	2011
Entity Information Line Items
Current Fiscal Year End Date	--12-31
Entity Central Index Key	0001172052
Entity Current Reporting Status	Yes
Entity Filer Category	Accelerated Filer
Entity Registrant Name	SAFETY INSURANCE GROUP INC
Entity Voluntary Filers	No
Entity Well Known Seasoned Issuer	No
Entity Common Stock Shares Outstanding		15,186,787

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Reference 1: http://www.xbrl.org/2003/role/presentationRef

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