XML 79 R51.htm IDEA: XBRL DOCUMENT
v3.19.3

Fair Value Measurements (Quantitative Disclosures of Fair Value Assets) (Details) - USD ($) $ in Thousands | Sep. 30, 2019 | Dec. 31, 2018
--- | --- | ---
AFS Fixed Income Securities |  | $ 5,930,237 | $ 5,273,100
Equity Securities |  | 79,213 | 147,639
Short-term investments |  | 326,121 | 323,864
Net Asset Value |  | 33,200 | 37,300
Fair Value, Measurements, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 5,930,237 | 5,273,100
Equity Securities |  | 79,213 | 147,639
Short-term investments |  | 326,121 | 323,864
Total assets measured at fair value |  | 6,335,571 | 5,744,603
U.S. Government and Government Agencies [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 120,900 | 121,310
Foreign Government [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 21,520 | 23,131
Obligations of States and Political Subdivisions [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 1,190,615 | 1,138,469
Corporate Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 1,838,927 | 1,617,408
Collateralized Loan Obligations and Other Asset Backed Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 768,383 | 717,362
Commercial Mortgage Backed Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 555,731 | 527,078
Residential Mortgage Backed Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 1,434,161 | 1,128,342
Common Stock [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
Equity Securities | [1] | 76,179 | 144,727
Preferred Stock [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
Equity Securities |  | 3,034 | 2,912
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
AFS Fixed Income Securities | [2] | 41,619 | 78,381
Equity Securities | [2] | 46,022 | 110,309
Short-term investments | [2] | 319,599 | 321,370
Total assets measured at fair value | [2] | 407,240 | 510,060
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | U.S. Government and Government Agencies [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
AFS Fixed Income Securities | [2] | 41,619 | 78,381
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Foreign Government [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
AFS Fixed Income Securities | [2] | 0 | 0
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Obligations of States and Political Subdivisions [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
AFS Fixed Income Securities | [2] | 0 | 0
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Corporate Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
AFS Fixed Income Securities | [2] | 0 | 0
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Collateralized Loan Obligations and Other Asset Backed Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
AFS Fixed Income Securities | [2] | 0 | 0
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Commercial Mortgage Backed Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
AFS Fixed Income Securities | [2] | 0 | 0
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Residential Mortgage Backed Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
AFS Fixed Income Securities | [2] | 0 | 0
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Common Stock [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
Equity Securities | [1],[2] | 42,988 | 107,397
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Preferred Stock [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
Equity Securities | [2] | 3,034 | 2,912
Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
AFS Fixed Income Securities | [2] | 5,852,389 | 5,187,310
Equity Securities | [2] | 0 | 0
Short-term investments | [2] | 6,522 | 2,494
Total assets measured at fair value | [2] | 5,858,911 | 5,189,804
Significant Other Observable Inputs (Level 2) [Member] | U.S. Government and Government Agencies [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
AFS Fixed Income Securities | [2] | 79,281 | 42,929
Significant Other Observable Inputs (Level 2) [Member] | Foreign Government [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
AFS Fixed Income Securities | [2] | 21,520 | 23,131
Significant Other Observable Inputs (Level 2) [Member] | Obligations of States and Political Subdivisions [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
AFS Fixed Income Securities | [2] | 1,190,615 | 1,138,469
Significant Other Observable Inputs (Level 2) [Member] | Corporate Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
AFS Fixed Income Securities | [2] | 1,822,577 | 1,617,408
Significant Other Observable Inputs (Level 2) [Member] | Collateralized Loan Obligations and Other Asset Backed Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
AFS Fixed Income Securities | [2] | 748,504 | 709,953
Significant Other Observable Inputs (Level 2) [Member] | Commercial Mortgage Backed Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
AFS Fixed Income Securities | [2] | 555,731 | 527,078
Significant Other Observable Inputs (Level 2) [Member] | Residential Mortgage Backed Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
AFS Fixed Income Securities | [2] | 1,434,161 | 1,128,342
Significant Other Observable Inputs (Level 2) [Member] | Common Stock [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
Equity Securities | [1],[2] | 0 | 0
Significant Other Observable Inputs (Level 2) [Member] | Preferred Stock [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
Equity Securities | [2] | 0 | 0
Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 36,229 | 7,409
Equity Securities |  | 0 | 0
Short-term investments |  | 0 | 0
Total assets measured at fair value |  | 36,229 | 7,409
Significant Unobservable Inputs (Level 3) [Member] | U.S. Government and Government Agencies [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 0 | 0
Significant Unobservable Inputs (Level 3) [Member] | Foreign Government [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 0 | 0
Significant Unobservable Inputs (Level 3) [Member] | Obligations of States and Political Subdivisions [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 0 | 0
Significant Unobservable Inputs (Level 3) [Member] | Corporate Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 16,350 | 0
Significant Unobservable Inputs (Level 3) [Member] | Collateralized Loan Obligations and Other Asset Backed Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 19,879 | 7,409
Significant Unobservable Inputs (Level 3) [Member] | Commercial Mortgage Backed Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 0 | 0
Significant Unobservable Inputs (Level 3) [Member] | Residential Mortgage Backed Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 0 | 0
Significant Unobservable Inputs (Level 3) [Member] | Common Stock [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
Equity Securities | [1] | 0 | 0
Significant Unobservable Inputs (Level 3) [Member] | Preferred Stock [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
Equity Securities |  | $ 0 | $ 0
[1] | Investments amounting to $33.2 million at September 30, 2019, and $37.3 million at December 31, 2018, were measured at fair value using net asset value per share (or its practical expedient) and have not been classified in the fair value hierarchy. The fair value amounts presented in this table are intended to permit reconciliation of the fair value hierarchy to total assets measured at fair value.
[2] | There were no transfers of securities between Level 1 and Level 2.

X
- Definition Net asset value of certain common stock investments that calculate net asset value per share. + References No definition available. + Details
Name: | sigi_NetAssetValue
Namespace Prefix: | sigi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258 + Details
Name: | us-gaap_AssetsFairValueDisclosure
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=109979856&loc=d3e26610-111562 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563 + Details
Name: | us-gaap_AvailableForSaleSecuritiesDebtSecurities
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 45 -Paragraph 1A -URI http://asc.fasb.org/extlink&oid=99377092&loc=SL75136599-209740 + Details
Name: | us-gaap_EquitySecuritiesFvNi
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of short-term investments classified as other. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.1(g)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.8) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.5) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 + Details
Name: | us-gaap_OtherShortTermInvestments
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Details
Name: | us-gaap_FairValueByMeasurementFrequencyAxis=us-gaap_FairValueMeasurementsRecurringMember
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: | us-gaap_InvestmentTypeAxis=us-gaap_CommonStockMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_InvestmentTypeAxis=us-gaap_NonredeemablePreferredStockMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel1Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel2Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |