XML 118 R91.htm IDEA: XBRL DOCUMENT
v3.8.0.1

Segment Information (Income From Continuing Operations, Before Federal Income Tax) (Details) - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2017 | Sep. 30, 2017 | Jun. 30, 2017 | Mar. 31, 2017 | Dec. 31, 2016 | Sep. 30, 2016 | Jun. 30, 2016 | Mar. 31, 2016 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Net investment income | $ 42,587 | $ 40,446 | $ 41,430 | $ 37,419 | $ 35,428 | $ 33,375 | $ 31,182 | $ 30,769 | $ 161,882 | $ 130,754 | $ 121,316
Realized investment gains (losses) |  |  |  |  |  |  |  |  | 6,359 | (4,937) | 13,171
Total investment revenues |  |  |  |  |  |  |  |  | 168,241 | 125,817 | 134,487
Tax on investment income |  |  |  |  |  |  |  |  | 45,588 | 30,621 | 32,090
Total investment income, after federal income tax |  |  |  |  |  |  |  |  | 122,653 | 95,196 | 102,397
Standard Commercial Lines [Member] |  |  |  |  |  |  |  |  |  |  |
Underwriting gain (loss), before federal income tax |  |  |  |  |  |  |  |  | 149,514 | 146,435 | 164,496
Underwriting gain (loss), after federal income tax |  |  |  |  |  |  |  |  | $ 97,184 | $ 95,183 | $ 106,923
Combined Ratio |  |  |  |  |  |  |  |  | 91.60% | 91.20% | 89.20%
Standard Personal Lines [Member] |  |  |  |  |  |  |  |  |  |  |
Underwriting gain (loss), before federal income tax |  |  |  |  |  |  |  |  | $ 11,104 | $ 12,419 | $ 1,336
Underwriting gain (loss), after federal income tax |  |  |  |  |  |  |  |  | $ 7,217 | $ 8,072 | $ 868
Combined Ratio |  |  |  |  |  |  |  |  | 96.20% | 95.60% | 99.50%
Excess And Surplus Operations [Member] |  |  |  |  |  |  |  |  |  |  |
Underwriting gain (loss), before federal income tax |  |  |  |  |  |  |  |  | $ (6,282) | $ (6,921) | $ (16,803)
Underwriting gain (loss), after federal income tax |  |  |  |  |  |  |  |  | $ (4,083) | $ (4,499) | $ (10,922)
Combined Ratio |  |  |  |  |  |  |  |  | 103.00% | 103.40% | 109.80%

X
- Definition Taxes incurred on investment income + References No definition available. + Details
Name: | sigi_Taxoninvestmentincome
Namespace Prefix: | sigi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition The income earned from investments in securities and other investments including dividends, interest, and returns on other investments, including realized gains/losses, after-tax + References No definition available. + Details
Name: | sigi_TotalInvestmentIncomeAfterFederalIncomeTax
Namespace Prefix: | sigi_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition The income earned from investments in securities and other investments including dividends, interest, and returns on other investments, including realized gains/losses. + References No definition available. + Details
Name: | sigi_TotalInvestmentRevenues
Namespace Prefix: | sigi_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition After-tax amount of net premiums earned and other insurance income less net loss and loss expenses incurred, loss adjustment expenses, policy acquisition costs and other underwriting expenses. + References No definition available. + Details
Name: | sigi_UnderwritingIncomeLossAfterFederalIncomeTaxes
Namespace Prefix: | sigi_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Net premiums earned and other insurance income less net loss and loss expenses incurred, loss adjustment expenses, policy acquisition costs and other underwriting expenses. + References No definition available. + Details
Name: | sigi_UnderwritingIncomeLossBeforeFederalIncomeTaxes
Namespace Prefix: | sigi_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Ratio of non-life losses and loss expenses, non-life acquisition expenses and non-life general and administrative expenses, to non-life net premiums earned. + References No definition available. + Details
Name: | us-gaap_CombinedRatio
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount, after investment expense, of income earned from investments in securities and real estate. Includes, but is not limited to, real estate investment, policy loans, dividends, and interest. Excludes realized gain (loss) on investments. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04(2)) -URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 235 -Section S99 -Paragraph 1 -Subparagraph (SX 210.12-16(Column G)) -URI http://asc.fasb.org/extlink&oid=34713774&loc=d3e574960-122915 + Details
Name: | us-gaap_NetInvestmentIncome
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition The net realized gains or losses on investments during the period, not including gains or losses on securities separately or otherwise categorized as trading, available-for-sale, or held-to-maturity. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04.3(a)) -URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 325 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=75031664&loc=d3e22529-158502 + Details
Name: | us-gaap_RealizedInvestmentGainsLosses
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Details
Name: | us-gaap_StatementBusinessSegmentsAxis=sigi_StandardCommercialLinesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_StatementBusinessSegmentsAxis=us-gaap_PropertyAndCasualtyPersonalInsuranceProductLineMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_StatementBusinessSegmentsAxis=sigi_ExcessAndSurplusOperationsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |