XML 31 R28.htm IDEA: XBRL DOCUMENT v2.4.1.9

Investments (Held-To-Maturity Securities Disclosure) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Dec. 31, 2014
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 295,859us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment	$ 317,179us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment
Net Unrealized Gains (Losses)	698sigi_HeldToMaturitySecuritiesNetUnrealizedGainsLosses	958sigi_HeldToMaturitySecuritiesNetUnrealizedGainsLosses
Carrying Value	296,557us-gaap_HeldToMaturitySecurities	318,137us-gaap_HeldToMaturitySecurities
Unrecognized Holding Gains	14,536us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain	15,824us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain
Unrecognized Holding Losses	0us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss	0us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
HTM, Fair Value	311,093us-gaap_HeldToMaturitySecuritiesFairValue	333,961us-gaap_HeldToMaturitySecuritiesFairValue
Held To Maturity securities average duration	1 year 8 months 8 days
Foreign Government [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	5,292us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment / us-gaap_DebtSecurityAxis = us-gaap_ForeignGovernmentDebtSecuritiesMember	5,292us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment / us-gaap_DebtSecurityAxis = us-gaap_ForeignGovernmentDebtSecuritiesMember
Net Unrealized Gains (Losses)	25sigi_HeldToMaturitySecuritiesNetUnrealizedGainsLosses / us-gaap_DebtSecurityAxis = us-gaap_ForeignGovernmentDebtSecuritiesMember	47sigi_HeldToMaturitySecuritiesNetUnrealizedGainsLosses / us-gaap_DebtSecurityAxis = us-gaap_ForeignGovernmentDebtSecuritiesMember
Carrying Value	5,317us-gaap_HeldToMaturitySecurities / us-gaap_DebtSecurityAxis = us-gaap_ForeignGovernmentDebtSecuritiesMember	5,339us-gaap_HeldToMaturitySecurities / us-gaap_DebtSecurityAxis = us-gaap_ForeignGovernmentDebtSecuritiesMember
Unrecognized Holding Gains	21us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain / us-gaap_DebtSecurityAxis = us-gaap_ForeignGovernmentDebtSecuritiesMember	55us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain / us-gaap_DebtSecurityAxis = us-gaap_ForeignGovernmentDebtSecuritiesMember
Unrecognized Holding Losses	0us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss / us-gaap_DebtSecurityAxis = us-gaap_ForeignGovernmentDebtSecuritiesMember	0us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss / us-gaap_DebtSecurityAxis = us-gaap_ForeignGovernmentDebtSecuritiesMember
HTM, Fair Value	5,338us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_DebtSecurityAxis = us-gaap_ForeignGovernmentDebtSecuritiesMember	5,394us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_DebtSecurityAxis = us-gaap_ForeignGovernmentDebtSecuritiesMember
Obligations of States and Political Subdivisions [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	264,788us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment / us-gaap_DebtSecurityAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember	285,301us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment / us-gaap_DebtSecurityAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember
Net Unrealized Gains (Losses)	1,706sigi_HeldToMaturitySecuritiesNetUnrealizedGainsLosses / us-gaap_DebtSecurityAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember	2,071sigi_HeldToMaturitySecuritiesNetUnrealizedGainsLosses / us-gaap_DebtSecurityAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember
Carrying Value	266,494us-gaap_HeldToMaturitySecurities / us-gaap_DebtSecurityAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember	287,372us-gaap_HeldToMaturitySecurities / us-gaap_DebtSecurityAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember
Unrecognized Holding Gains	10,536us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain / us-gaap_DebtSecurityAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember	11,760us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain / us-gaap_DebtSecurityAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember
Unrecognized Holding Losses	0us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss / us-gaap_DebtSecurityAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember	0us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss / us-gaap_DebtSecurityAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember
HTM, Fair Value	277,030us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_DebtSecurityAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember	299,132us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_DebtSecurityAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember
Corporate Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	18,532us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment / us-gaap_DebtSecurityAxis = us-gaap_CorporateDebtSecuritiesMember	18,899us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment / us-gaap_DebtSecurityAxis = us-gaap_CorporateDebtSecuritiesMember
Net Unrealized Gains (Losses)	(269)sigi_HeldToMaturitySecuritiesNetUnrealizedGainsLosses / us-gaap_DebtSecurityAxis = us-gaap_CorporateDebtSecuritiesMember	(273)sigi_HeldToMaturitySecuritiesNetUnrealizedGainsLosses / us-gaap_DebtSecurityAxis = us-gaap_CorporateDebtSecuritiesMember
Carrying Value	18,263us-gaap_HeldToMaturitySecurities / us-gaap_DebtSecurityAxis = us-gaap_CorporateDebtSecuritiesMember	18,626us-gaap_HeldToMaturitySecurities / us-gaap_DebtSecurityAxis = us-gaap_CorporateDebtSecuritiesMember
Unrecognized Holding Gains	2,936us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain / us-gaap_DebtSecurityAxis = us-gaap_CorporateDebtSecuritiesMember	2,796us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain / us-gaap_DebtSecurityAxis = us-gaap_CorporateDebtSecuritiesMember
Unrecognized Holding Losses	0us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss / us-gaap_DebtSecurityAxis = us-gaap_CorporateDebtSecuritiesMember	0us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss / us-gaap_DebtSecurityAxis = us-gaap_CorporateDebtSecuritiesMember
HTM, Fair Value	21,199us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_DebtSecurityAxis = us-gaap_CorporateDebtSecuritiesMember	21,422us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_DebtSecurityAxis = us-gaap_CorporateDebtSecuritiesMember
Asset-Backed Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2,483us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment / us-gaap_DebtSecurityAxis = us-gaap_AssetBackedSecuritiesMember	2,818us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment / us-gaap_DebtSecurityAxis = us-gaap_AssetBackedSecuritiesMember
Net Unrealized Gains (Losses)	(378)sigi_HeldToMaturitySecuritiesNetUnrealizedGainsLosses / us-gaap_DebtSecurityAxis = us-gaap_AssetBackedSecuritiesMember	(455)sigi_HeldToMaturitySecuritiesNetUnrealizedGainsLosses / us-gaap_DebtSecurityAxis = us-gaap_AssetBackedSecuritiesMember
Carrying Value	2,105us-gaap_HeldToMaturitySecurities / us-gaap_DebtSecurityAxis = us-gaap_AssetBackedSecuritiesMember	2,363us-gaap_HeldToMaturitySecurities / us-gaap_DebtSecurityAxis = us-gaap_AssetBackedSecuritiesMember
Unrecognized Holding Gains	375us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain / us-gaap_DebtSecurityAxis = us-gaap_AssetBackedSecuritiesMember	460us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain / us-gaap_DebtSecurityAxis = us-gaap_AssetBackedSecuritiesMember
Unrecognized Holding Losses	0us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss / us-gaap_DebtSecurityAxis = us-gaap_AssetBackedSecuritiesMember	0us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss / us-gaap_DebtSecurityAxis = us-gaap_AssetBackedSecuritiesMember
HTM, Fair Value	2,480us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_DebtSecurityAxis = us-gaap_AssetBackedSecuritiesMember	2,823us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_DebtSecurityAxis = us-gaap_AssetBackedSecuritiesMember
Commercial Mortgage Backed Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	4,764us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment / us-gaap_DebtSecurityAxis = us-gaap_CommercialMortgageBackedSecuritiesMember	4,869us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment / us-gaap_DebtSecurityAxis = us-gaap_CommercialMortgageBackedSecuritiesMember
Net Unrealized Gains (Losses)	(386)sigi_HeldToMaturitySecuritiesNetUnrealizedGainsLosses / us-gaap_DebtSecurityAxis = us-gaap_CommercialMortgageBackedSecuritiesMember	(432)sigi_HeldToMaturitySecuritiesNetUnrealizedGainsLosses / us-gaap_DebtSecurityAxis = us-gaap_CommercialMortgageBackedSecuritiesMember
Carrying Value	4,378us-gaap_HeldToMaturitySecurities / us-gaap_DebtSecurityAxis = us-gaap_CommercialMortgageBackedSecuritiesMember	4,437us-gaap_HeldToMaturitySecurities / us-gaap_DebtSecurityAxis = us-gaap_CommercialMortgageBackedSecuritiesMember
Unrecognized Holding Gains	668us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain / us-gaap_DebtSecurityAxis = us-gaap_CommercialMortgageBackedSecuritiesMember	753us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain / us-gaap_DebtSecurityAxis = us-gaap_CommercialMortgageBackedSecuritiesMember
Unrecognized Holding Losses	0us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss / us-gaap_DebtSecurityAxis = us-gaap_CommercialMortgageBackedSecuritiesMember	0us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss / us-gaap_DebtSecurityAxis = us-gaap_CommercialMortgageBackedSecuritiesMember
HTM, Fair Value	$ 5,046us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_DebtSecurityAxis = us-gaap_CommercialMortgageBackedSecuritiesMember	$ 5,190us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_DebtSecurityAxis = us-gaap_CommercialMortgageBackedSecuritiesMember

X

- Definition

Held to maturity securities average duration.

+ References

No definition available.

+ Details

Name:	sigi_HeldToMaturitySecuritiesAverageDuration
Namespace Prefix:	sigi_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

X

- Definition

The remaining aggregate increase (decrease) in the market value of held-to-maturity securities either (i) prior to the securities' transfer to a held-to-maturity designation or (ii) resulting from a non-credit other-than-temporary impairment.

+ References

No definition available.

+ Details

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

X

- Definition

Amount after other than temporary impairment (OTTI) accretion, of investments in debt securities classified as held-to-maturity.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 25

-Paragraph 1

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=51813488&loc=d3e22054-111558



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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 5

-Subparagraph (f)

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563



Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 5

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563



Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-03.1)

-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910



Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-03.6)

-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

Name:	us-gaap_HeldToMaturitySecurities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Definition

Amount of accumulated unrealized gain on investments in debt securities classified as held-to-maturity.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 5

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563

+ Details

Name:	us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Amount of accumulated unrealized loss on investments in debt securities classified as held-to-maturity.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 5

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563

+ Details

Name:	us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Definition

Amount before other than temporary impairment (OTTI) accretion, of investments in debt securities classified as held-to-maturity.

+ References

No definition available.

+ Details

Name:	us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Definition

Fair value of investments in debt securities classified as held-to-maturity.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 5

-Subparagraph (aa)

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563



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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 5

-Subparagraph (f)

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563



Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 5

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563

+ Details

Name:	us-gaap_HeldToMaturitySecuritiesFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Details

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

X

- Details

Name:	us-gaap_DebtSecurityAxis=us-gaap_AssetBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_DebtSecurityAxis=us-gaap_CommercialMortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_DebtSecurityAxis=us-gaap_CorporateDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_DebtSecurityAxis=us-gaap_ForeignGovernmentDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_DebtSecurityAxis=us-gaap_USStatesAndPoliticalSubdivisionsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: