XML 131 R102.htm IDEA: XBRL DOCUMENT v2.4.1.9

Retirement Plans (Plan's Allocated Target and Ranges and Actual Weighted Average Asset Allocation by Investment Categories) (Details)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Actual percentage	100.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations	100.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations
Maximum percentage of equity security value of total plans portfolio value that is allowed	8.00%sigi_MaximumPercentageOfAnySecurityValueOfTotalPlansPortfolioVaueThatIsAllowed
Maximum percentage of any corporation outstanding shares that is allowed for retirement income plan equity portfolio	5.00%sigi_MaximumPercentageOfAnyCorporationOutstandingSharesThatIsAllowedForRetirementIncomePlan
Global Asset Allocation [Member]
Actual percentage	11.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = sigi_GlobalAssetAllocationMember		12.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = sigi_GlobalAssetAllocationMember
Range minimum	0.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocationsRangeMinimum / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = sigi_GlobalAssetAllocationMember
Range maximum	0.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocationsRangeMaximum / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = sigi_GlobalAssetAllocationMember
Private Equity Funds [Member]
Actual percentage	4.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PrivateEquityFundsMember	5.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PrivateEquityFundsMember
Range minimum	0.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocationsRangeMinimum / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PrivateEquityFundsMember
Range maximum	0.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocationsRangeMaximum / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PrivateEquityFundsMember
Cash And Short Term Investments [Member]
Actual percentage	1.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = sigi_CashAndShortTermInvestmentsMember	1.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = sigi_CashAndShortTermInvestmentsMember
Range minimum	0.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocationsRangeMinimum / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = sigi_CashAndShortTermInvestmentsMember
Range maximum	0.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocationsRangeMaximum / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = sigi_CashAndShortTermInvestmentsMember
Extended Duration Fixed Maturity [Member]
Actual percentage	59.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = sigi_TotalLongDurationFixedIncomeMember	55.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = sigi_TotalLongDurationFixedIncomeMember
Range minimum	55.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocationsRangeMinimum / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = sigi_TotalLongDurationFixedIncomeMember
Range maximum	100.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocationsRangeMaximum / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = sigi_TotalLongDurationFixedIncomeMember
Global Equity [Member]
Actual percentage	25.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = sigi_GlobalEquityMember	27.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = sigi_GlobalEquityMember
Range minimum	0.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocationsRangeMinimum / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = sigi_GlobalEquityMember
Range maximum	45.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocationsRangeMaximum / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = sigi_GlobalEquityMember

X

- Definition

The maximum percentage of outstanding shares of any one corporation that is allowed for the Retirement Income Plan.

+ References

No definition available.

+ Details

Name:	sigi_MaximumPercentageOfAnyCorporationOutstandingSharesThatIsAllowedForRetirementIncomePlan
Namespace Prefix:	sigi_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

X

- Definition

The maximum percentage of any individual security value of the total Retirement Income Plan's portfolio value that is allowed.

+ References

No definition available.

+ Details

Name:	sigi_MaximumPercentageOfAnySecurityValueOfTotalPlansPortfolioVaueThatIsAllowed
Namespace Prefix:	sigi_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

X

- Definition

Target allocation maximum percentage of investments of this type to total plan assets presented on a weighted average basis.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (d)(i)

-URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 20

-Section 50

-Paragraph 5

-Subparagraph (c)(i)

-URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e2446-114920

+ Details

Name:	us-gaap_DefinedBenefitPlanTargetPlanAssetAllocationsRangeMaximum
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Target allocation minimum percentage of investments of this type to total plan assets presented on a weighted average basis.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (d)(i)

-URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 20

-Section 50

-Paragraph 5

-Subparagraph (c)(i)

-URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e2446-114920

+ Details

Name:	us-gaap_DefinedBenefitPlanTargetPlanAssetAllocationsRangeMinimum
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Percentage of fair value of investments (categorized by debt securities, equity securities, real estate and other plan assets) to the fair value of plan assets held.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 20

-Section 50

-Paragraph 5

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e2446-114920



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (d)(1)

-URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920

+ Details

Name:	us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

X

- Details

Name:	us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis=sigi_CashAndShortTermInvestmentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis=sigi_GlobalAssetAllocationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis=sigi_GlobalEquityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis=sigi_TotalLongDurationFixedIncomeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis=us-gaap_PrivateEquityFundsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: