XML 131 R103.htm IDEA: XBRL DOCUMENT v3.10.0.1

Retirement Plans (Plan's Allocated Target Ranges and Actual Weighted Average Asset Allocation by Investment Categories) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017
Plan Assets Redeployed into Risk Seeking Investments		$ 49
Actual percentage		100.00%	100.00%
Percent of Plan Assets Redeployed into Risk Seeking Investments		15.00%
Percent Hedge Against Projected Benefit Obligation		50.00%
Return Seeking Assets [Member]
Actual percentage		43.00%	58.00%
Liability Hedging Assets [Member]
Actual percentage		38.00%	40.00%
Short-term Investments Excluding Redeployment[Member]
Actual percentage	[1]	5.00%
Short-term Investments [Member]
Actual percentage	[1]	19.00%	2.00%
Minimum [Member] | Return Seeking Assets [Member]
Defined Benefit Plan, Plan Assets, Investment Policy and Strategy, Description		0.15
Minimum [Member] | Liability Hedging Assets [Member]
Defined Benefit Plan, Plan Assets, Investment Policy and Strategy, Description		0.35
Minimum [Member] | Short-term Investments [Member]
Defined Benefit Plan, Plan Assets, Investment Policy and Strategy, Description		0
Maximum [Member] | Return Seeking Assets [Member]
Defined Benefit Plan, Plan Assets, Investment Policy and Strategy, Description		0.7
Maximum [Member] | Liability Hedging Assets [Member]
Defined Benefit Plan, Plan Assets, Investment Policy and Strategy, Description		0.75
Maximum [Member] | Short-term Investments [Member]
Defined Benefit Plan, Plan Assets, Investment Policy and Strategy, Description		0.02
[1] The actual percentage of assets allocated to short-term investments is higher than our target as of December 31, 2018 due to portfolio rebalancing initiatives that are expected to be completed during 2019. As part of this rebalancing initiative, $49 million, or 15%, of plan assets were redeployed into risk seeking assets in January 2019. Excluding the impact of this redeployment, our short-term investments represented 5% of plan assets at December 31, 2018.

X

- Definition

Percent hedge provided against the projected benefit obligation.

+ References

No definition available.

+ Details

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

X

- Definition

The percentage of Plan assets that were redeployed from short term investments into risk seeking investments.

+ References

No definition available.

+ Details

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

X

- Definition

Amount of Plan assets that were redeployed from short-term investments into risk-seeking investments.

+ References

No definition available.

+ Details

Name:	sigi_PlanAssetsRedeployedintoRiskSeekingInvestments
Namespace Prefix:	sigi_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Definition

Description of investment policy and strategy for defined benefit plan asset. Includes, but is not limited to, investment allocation decision and factor pertinent to understanding investment policy and strategy for plan asset. Factor includes, but is not limited to, investment goal, risk management practice, permitted and prohibited investment including use of derivative, diversification, and relationship between plan asset and obligation.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 55
-Paragraph 17
-URI http://asc.fasb.org/extlink&oid=112275985&loc=d3e4179-114921

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)(i)
-URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920

+ Details

Name:	us-gaap_DefinedBenefitPlanInvestmentPoliciesAndStrategiesNarrativeDescription
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Percentage of actual investment allocation to total plan assets. Includes, but is not limited to, percentage on weighted-average basis if more than one plan.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)(i)
-URI http://asc.fasb.org/extlink&oid=108410482&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_DefinedBenefitPlanByPlanAssetCategoriesAxis=sigi_ReturnSeekingAssetsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=sigi_LiabilityHedgingAssetsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=sigi_ShortTermInvestmentsExcludingRedeploymentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=us-gaap_ShortTermInvestmentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: