XML 48 R36.htm IDEA: XBRL DOCUMENT v3.8.0.1

Investments (Fair Value and Gross Pre-Tax Net Unrealized Unrecognized Loss of Securities By Length of Time) (Details) - USD ($) $ in Thousands		Mar. 31, 2018	Dec. 31, 2017
Percent Of Securities of Amortized Cost In An Unrealized Unrecognized Loss Position		1.00%	1.00%
AFS Fair Value, Less than 12 months		$ 2,744,048	$ 878,126
AFS Unrealized Losses, Less than 12 months	[1]	(33,675)	(4,961)
AFS Fair Value, 12 months or longer		16,520	35,686
AFS Unrealized Losses, 12 months or longer	[1]	(536)	(397)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value		2,760,568	913,812
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	[1]	(34,211)	(5,358)
Held-to-maturity Securities [Member]
Unrealized Unrecognized Loss		0	100
AFS Fixed Income Securities [Member]
AFS Fair Value, Less than 12 months			869,566
AFS Unrealized Losses, Less than 12 months	[1]		(4,664)
AFS Fair Value, 12 months or longer			35,686
AFS Unrealized Losses, 12 months or longer	[1]		(397)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value			905,252
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	[1]		(5,061)
AFS Fixed Income Securities [Member] | U.S. Government and Government Agencies [Member]
AFS Fair Value, Less than 12 months		27,084	23,516
AFS Unrealized Losses, Less than 12 months	[1]	(624)	(233)
AFS Fair Value, 12 months or longer		0	250
AFS Unrealized Losses, 12 months or longer	[1]	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value		27,084	23,766
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	[1]	(624)	(233)
AFS Fixed Income Securities [Member] | Foreign Government [Member]
AFS Fair Value, Less than 12 months		6,367	1,481
AFS Unrealized Losses, Less than 12 months	[1]	(85)	(11)
AFS Fair Value, 12 months or longer		0	0
AFS Unrealized Losses, 12 months or longer	[1]	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value		6,367	1,481
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	[1]	(85)	(11)
AFS Fixed Income Securities [Member] | Obligations of States and Political Subdivisions [Member]
AFS Fair Value, Less than 12 months		344,037	107,514
AFS Unrealized Losses, Less than 12 months	[1]	(4,489)	(422)
AFS Fair Value, 12 months or longer		3,416	14,139
AFS Unrealized Losses, 12 months or longer	[1]	(147)	(160)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value		347,453	121,653
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	[1]	(4,636)	(582)
AFS Fixed Income Securities [Member] | Corporate Securities [Member]
AFS Fair Value, Less than 12 months		910,974	238,326
AFS Unrealized Losses, Less than 12 months	[1]	(13,348)	(1,744)
AFS Fair Value, 12 months or longer		1,677	3,228
AFS Unrealized Losses, 12 months or longer	[1]	(68)	(18)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value		912,651	241,554
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	[1]	(13,416)	(1,762)
AFS Fixed Income Securities [Member] | Collateralized Loan Obligations and Other Asset Backed Securities [Member]
AFS Fair Value, Less than 12 months		384,113	74,977
AFS Unrealized Losses, Less than 12 months	[1]	(992)	(196)
AFS Fair Value, 12 months or longer		1,095	1,655
AFS Unrealized Losses, 12 months or longer	[1]	(7)	(6)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value		385,208	76,632
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	[1]	(999)	(202)
AFS Fixed Income Securities [Member] | Commercial Mortgage Backed Securities [Member]
AFS Fair Value, Less than 12 months		321,980	154,267
AFS Unrealized Losses, Less than 12 months	[1]	(4,715)	(773)
AFS Fair Value, 12 months or longer		0	5,214
AFS Unrealized Losses, 12 months or longer	[1]	0	(68)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value		321,980	159,481
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	[1]	(4,715)	(841)
AFS Fixed Income Securities [Member] | Residential Mortgage Backed Securities [Member]
AFS Fair Value, Less than 12 months		749,493	269,485
AFS Unrealized Losses, Less than 12 months	[1]	(9,422)	(1,285)
AFS Fair Value, 12 months or longer		10,332	11,200
AFS Unrealized Losses, 12 months or longer	[1]	(314)	(145)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value		759,825	280,685
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	[1]	$ (9,736)	(1,430)
Equity Securities [Member]
AFS Fair Value, Less than 12 months			8,560
AFS Unrealized Losses, Less than 12 months	[1]		(297)
AFS Fair Value, 12 months or longer			0
AFS Unrealized Losses, 12 months or longer	[1]		0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value			8,560
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	[1]		(297)
Equity Securities [Member] | Common Stock [Member]
AFS Fair Value, Less than 12 months			4,727
AFS Unrealized Losses, Less than 12 months	[1]		(226)
AFS Fair Value, 12 months or longer			0
AFS Unrealized Losses, 12 months or longer	[1]		0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value			4,727
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	[1]		(226)
Equity Securities [Member] | Preferred Stock [Member]
AFS Fair Value, Less than 12 months			3,833
AFS Unrealized Losses, Less than 12 months	[1]		(71)
AFS Fair Value, 12 months or longer			0
AFS Unrealized Losses, 12 months or longer	[1]		0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value			3,833
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	[1]		$ (71)
[1] Gross unrealized losses include non-OTTI unrealized amounts and OTTI losses recognized in AOCI.

X

- Definition

This item contains disclosure of the percent of available-for-sale and held-to-maturity securities determined to be in an unrealized/unrecognized loss position.

+ References

No definition available.

+ Details

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

X

- Definition

This item represents the sum of (i) the excess of amortized cost over fair value for available-for-sale securities that are in a loss position and (ii) the market fluctuations on held-to-maturity securities from the later of (a) the date a security is designated as held-to-maturity; or (b) the date that an other-than-temporary impairment charge is recognized on a held-to-maturity security for securities that are in an unrecognized loss position.

+ References

No definition available.

+ Details

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

X

- Definition

Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for twelve months or longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Definition

Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale in a continuous loss position.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

+ Details

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

X

- Definition

This item represents the aggregate fair value of investments in debt and equity securities in an unrealized loss position which are categorized neither as held-to-maturity nor trading securities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

+ Details

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

X

- Definition

Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for less than twelve months.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Definition

This item represents the aggregate fair value of investments in debt and equity securities categorized neither as held-to-maturity nor trading securities that have been in a continuous unrealized loss position for less than twelve months.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

+ Details

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

X

- Definition

This item represents the aggregate fair value of investments in debt and equity securities categorized neither as held-to-maturity nor trading securities that have been in a continuous unrealized loss position for twelve months or longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

+ Details

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

X

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_HeldtomaturitySecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis=sigi_AFSFixedIncomeSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_USTreasuryAndGovernmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

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

X

- Details

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

X

- Details

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

X

- Details

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

X

- Details

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

X

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_ResidentialMortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis=us-gaap_EquitySecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_CommonStockMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_NonredeemablePreferredStockMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: