XML 127 R100.htm IDEA: XBRL DOCUMENT
v3.3.1.900

Retirement Plans (Plan's Allocated Target and Ranges and Actual Weighted Average Asset Allocation by Investment Categories) (Details) | 12 Months Ended
--- | ---
Dec. 31, 2015 | Dec. 31, 2014
Actual percentage | 100.00% | 100.00%
Range minimum | 0.00% |
Range maximum |  | 0.00%
Maximum percentage of equity security value of total plans portfolio value that is allowed | 8.00% |
Maximum percentage of any corporation outstanding shares that is allowed for retirement income plan equity portfolio | 5.00% |
Extended Duration Fixed Maturity [Member] |  |
Actual percentage | 60.00% | 59.00%
Range minimum | 55.00% |
Range maximum | 100.00% |
Global Equity [Member] |  |
Actual percentage | 36.00% | 25.00%
Range minimum | 0.00% |
Range maximum | 45.00% |
Global Asset Allocation [Member] |  |
Actual percentage | 0.00% | 11.00%
Range minimum | 0.00% |
Range maximum | 0.00% |
Private Equity Funds [Member] |  |
Actual percentage | 3.00% | 4.00%
Range minimum | 0.00% |
Range maximum | 0.00% |
Cash And Short Term Investments [Member] |  |
Actual percentage | 1.00% | 1.00%
Range minimum | 0.00% |
Range maximum | 0.00% |

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 5 -Subparagraph (c)(i) -URI http://asc.fasb.org/extlink&oid=58740216&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)(i) -URI http://asc.fasb.org/extlink&oid=58740216&loc=d3e1928-114920 + 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=58740216&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=58740216&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=58740216&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=58740216&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_TotalLongDurationFixedIncomeMember
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_GlobalAssetAllocationMember
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: |

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