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Indebtedness (Covenants in the Line of Credit) (Details) - USD ($) | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
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Debt Disclosure [Abstract] |  |  |  |
Consolidated net worth, required |  | $ 1,200,000,000 |  |
Statutory surplus, required |  | $ 750,000,000 |  |
Debt-to-capitalization ratio, required | [1] | 35.00% |  |
Consolidated net worth, actual |  | $ 1,712,957,000 | $ 1,531,370,000 | $ 1,398,041,000
Statutory surplus, actual |  | $ 1,672,900,000 | $ 1,583,800,000 |
Debt-to-capitalization ratio, actual | [1] | 20.50% |  |
[1] | Calculated in accordance with Line of Credit agreement.

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- Definition This element represents the debt-to-capitalization ratio which is calculated by taking the entity's debt and dividing by the sum of the entity's debt and stockholders' equity as of the balance sheet date. + References No definition available. + Details
Name: | sigi_DebtToCapitalizationRatio
Namespace Prefix: | sigi_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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- Definition The covenant amount required for the line of credit for consolidated net worth as of the balance sheet date. + References No definition available. + Details
Name: | sigi_LineOfCreditConsolidatedNetWorthRequirement
Namespace Prefix: | sigi_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition The maximum covenant percentage required for the line of credit for the debt-to-capitalization ratio as of the balance sheet date. + References No definition available. + Details
Name: | sigi_LineOfCreditDebtToCapitalizationRatioRequirement
Namespace Prefix: | sigi_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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- Definition The minimum covenant amount required for the line of credit for statutory surplus as of the balance sheet date. + References No definition available. + Details
Name: | sigi_LineOfCreditStatutorySurplusRequirement
Namespace Prefix: | sigi_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- References No definition available. + Details
Name: | us-gaap_DebtDisclosureAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition The amount of statutory capital and surplus (stockholders' equity) as of the balance sheet date using prescribed or permitted statutory accounting practices (rather than GAAP, if different) of the state or country. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(a)(23)(c)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6486098&loc=d3e24708-158529 + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Total of all stockholders' equity (deficit) items, net of receivables from officers, directors, owners, and affiliates of the entity which are attributable to the parent. The amount of the economic entity's stockholders' equity attributable to the parent excludes the amount of stockholders' equity which is allocable to that ownership interest in subsidiary equity which is not attributable to the parent (noncontrolling interest, minority interest). This excludes temporary equity and is sometimes called permanent equity. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(30)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SAB Topic 4.E) -URI http://asc.fasb.org/extlink&oid=27010918&loc=d3e74512-122707 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(31)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(29)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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Balance Type: | credit
Period Type: | instant