XML 48 R36.htm IDEA: XBRL DOCUMENT v3.19.1

Investments (Fair Value and Gross Pre-Tax Net Unrealized Loss of Securities By Length of Time) (Details) - USD ($) $ in Thousands		Mar. 31, 2019	Dec. 31, 2018
Percent Of Securities of Amortized Cost In An Unrealized Unrecognized Loss Position		1.00%	2.00%
AFS Fixed Income Securities [Member]
AFS Fair Value, Less than 12 months		$ 688,988	$ 2,142,433
AFS Unrealized Losses, Less than 12 months	[1]	(7,889)	(36,948)
AFS Fair Value, 12 months or longer		438,135	275,174
AFS Unrealized Losses, 12 months or longer	[1]	(6,399)	(8,119)
AFS Fair Value		1,127,123	2,417,607
AFS Unrealized Losses	[1]	(14,288)	(45,067)
AFS Fixed Income Securities [Member] | U.S. Government and Government Agencies [Member]
AFS Fair Value, Less than 12 months		13,443	6,693
AFS Unrealized Losses, Less than 12 months	[1]	(41)	(174)
AFS Fair Value, 12 months or longer		25,774	23,163
AFS Unrealized Losses, 12 months or longer	[1]	(300)	(418)
AFS Fair Value		39,217	29,856
AFS Unrealized Losses	[1]	(341)	(592)
AFS Fixed Income Securities [Member] | Foreign Government [Member]
AFS Fair Value, Less than 12 months		0	12,208
AFS Unrealized Losses, Less than 12 months	[1]	0	(93)
AFS Fair Value, 12 months or longer		3,509	1,482
AFS Unrealized Losses, 12 months or longer	[1]	(11)	(14)
AFS Fair Value		3,509	13,690
AFS Unrealized Losses	[1]	(11)	(107)
AFS Fixed Income Securities [Member] | Obligations of States and Political Subdivisions [Member]
AFS Fair Value, Less than 12 months		15,257	196,798
AFS Unrealized Losses, Less than 12 months	[1]	(71)	(2,074)
AFS Fair Value, 12 months or longer		21,676	42,821
AFS Unrealized Losses, 12 months or longer	[1]	(126)	(557)
AFS Fair Value		36,933	239,619
AFS Unrealized Losses	[1]	(197)	(2,631)
AFS Fixed Income Securities [Member] | Corporate Securities [Member]
AFS Fair Value, Less than 12 months		158,607	1,041,952
AFS Unrealized Losses, Less than 12 months	[1]	(2,783)	(23,649)
AFS Fair Value, 12 months or longer		160,121	78,953
AFS Unrealized Losses, 12 months or longer	[1]	(3,330)	(4,316)
AFS Fair Value		318,728	1,120,905
AFS Unrealized Losses	[1]	(6,113)	(27,965)
AFS Fixed Income Securities [Member] | Collateralized Loan Obligations and Other Asset Backed Securities [Member]
AFS Fair Value, Less than 12 months		366,775	516,106
AFS Unrealized Losses, Less than 12 months	[1]	(4,231)	(6,750)
AFS Fair Value, 12 months or longer		79,489	16,800
AFS Unrealized Losses, 12 months or longer	[1]	(1,152)	(193)
AFS Fair Value		446,264	532,906
AFS Unrealized Losses	[1]	(5,383)	(6,943)
AFS Fixed Income Securities [Member] | Commercial Mortgage Backed Securities [Member]
AFS Fair Value, Less than 12 months		112,025	229,338
AFS Unrealized Losses, Less than 12 months	[1]	(505)	(2,548)
AFS Fair Value, 12 months or longer		63,774	66,294
AFS Unrealized Losses, 12 months or longer	[1]	(310)	(1,200)
AFS Fair Value		175,799	295,632
AFS Unrealized Losses	[1]	(815)	(3,748)
AFS Fixed Income Securities [Member] | Residential Mortgage Backed Securities [Member]
AFS Fair Value, Less than 12 months		22,881	139,338
AFS Unrealized Losses, Less than 12 months	[1]	(258)	(1,660)
AFS Fair Value, 12 months or longer		83,792	45,661
AFS Unrealized Losses, 12 months or longer	[1]	(1,170)	(1,421)
AFS Fair Value		106,673	184,999
AFS Unrealized Losses	[1]	$ (1,428)	$ (3,081)
[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

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

No definition available.

+ 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://fasb.org/us-gaap/role/ref/legacyRef
-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_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

No definition available.

+ 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

No definition available.

+ 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

No definition available.

+ 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

No definition available.

+ Details

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

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: