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: |