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Reinsurance (Total Reinsurance Balances Segregated By Reinsurer) (Details) - USD ($) $ in Thousands | 12 Months Ended |  |  |
--- | --- | --- | --- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
Terrorism Risk Insurance Program Companys Estimated Deductible |  | $ 323,000 |  |  |
Terrorism Risk Insurance Program Reimbursed Percentage Maximum |  | 82.00% |  |  |
Terrorism Risk Insurance Program Annual Cap |  | $ 100,000,000 |  |  |
Terrorism Risk Insurance Program Insurer Retainer |  | 18.00% |  |  |
Annual Terrorism Risk Insurance Program Federal Share Reduction |  | 1.00% |  |  |
Terrorism Risk Insurance Program Reimbursed Percentage Minimum |  | 80.00% |  |  |
Allowance For Reinsurance Recoverable |  | $ 14,600 | $ 11,480 | $ 10,122 | $ 11,037
Total reinsurance recoverables |  | 594,832 | 621,537 |  |
Total prepaid reinsurance premiums |  | 153,493 | 146,282 |  |
Net unsecured reinsurance balances |  | 748,325 | 767,819 |  |
Less: collateral |  | (122,413) | (113,763) |  |
National Flood Insurance Program [Member] |  |  |  |  |
Net unsecured reinsurance balances | [1] | $ 204,161 | $ 211,181 |  |
% of Net Unsecured Reinsurance | [1] | 27.00% | 27.00% |  |
NJ Unsatisfied Claim Judgment Fund [Member] |  |  |  |  |
Net unsecured reinsurance balances | [1] | $ 62,947 | $ 65,574 |  |
% of Net Unsecured Reinsurance | [1] | 9.00% | 9.00% |  |
Other Federal And State Pools [Member] |  |  |  |  |
Net unsecured reinsurance balances | [1] | $ 3,634 | $ 3,227 |  |
% of Net Unsecured Reinsurance | [1] | 0.00% | 0.00% |  |
Total Federal And State Pools [Member] |  |  |  |  |
Net unsecured reinsurance balances | [1] | $ 270,742 | $ 279,982 |  |
% of Net Unsecured Reinsurance | [1] | 36.00% | 36.00% |  |
Remaining Unsecured Reinsurance [Member] |  |  |  |  |
Net unsecured reinsurance balances |  | $ 477,583 | $ 487,837 |  |
% of Net Unsecured Reinsurance |  | 64.00% | 64.00% |  |
Munich Re Group [Member] |  |  |  |  |
Net unsecured reinsurance balances |  | $ 117,460 | $ 119,520 |  |
% of Net Unsecured Reinsurance |  | 16.00% | 16.00% |  |
Hanover Ruckversicherungs AG [Member] |  |  |  |  |
Net unsecured reinsurance balances |  | $ 101,652 | $ 106,298 |  |
% of Net Unsecured Reinsurance |  | 14.00% | 13.00% |  |
AXIS Reinsurance Company [Member] |  |  |  |  |
Net unsecured reinsurance balances |  | $ 62,396 | $ 59,737 |  |
% of Net Unsecured Reinsurance |  | 8.00% | 8.00% |  |
Swiss Re Group [Member] |  |  |  |  |
Net unsecured reinsurance balances |  | $ 40,772 | $ 50,494 |  |
% of Net Unsecured Reinsurance |  | 5.00% | 7.00% |  |
Partner Reinsurance Company Of The US [Member] |  |  |  |  |
Net unsecured reinsurance balances |  | $ 16,925 | $ 21,125 |  |
% of Net Unsecured Reinsurance |  | 2.00% | 3.00% |  |
All Other Reinsurers [Member] |  |  |  |  |
Net unsecured reinsurance balances |  | $ 138,378 | $ 130,663 |  |
% of Net Unsecured Reinsurance |  | 19.00% | 17.00% |  |
Remaining Unsecured Reinsurance, Net of Collateral [Member] |  |  |  |  |
Net unsecured reinsurance balances |  | $ 355,170 | $ 374,074 |  |
Allowance for Reinsurance Recoverable [Member] |  |  |  |  |
Allowance For Reinsurance Recoverable |  | $ 4,600 | $ 5,500 |  |
[1] | Considered to have minimal risk of default.

X
- Definition This element represents the amount collateralized by the entity's reinsurers. The amount includes letters of credit, trust funds, and funds withheld. + References No definition available. + Details
Name: | sigi_CollateralOfReinsuranceRecoverables
Namespace Prefix: | sigi_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition This element represents the percentage of the total net unsecured reinsurance balance. + References No definition available. + Details
Name: | sigi_PercentageOfNetUnsecuredReinsurance
Namespace Prefix: | sigi_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition This element represents the known and estimated amount of reinsurance recoverables and the unexpired portion of premiums ceded on policies in force, less all federal and state pools, as of the balance sheet date. + References No definition available. + Details
Name: | sigi_RemainingReinsuranceBalances
Namespace Prefix: | sigi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition The annual cap limiting the amount of aggregate subject losses for participating insurers under the Terrorism Risk Insurance Program. + References No definition available. + Details
Name: | sigi_TerrorismRiskInsuranceProgramAnnualCap
Namespace Prefix: | sigi_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition The Company's estimated deductible for the year following the date of this report under the Terrorism Risk Insurance Program. + References No definition available. + Details
Name: | sigi_TerrorismRiskInsuranceProgramCompanysEstimatedDeductible
Namespace Prefix: | sigi_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition The annual percentage decrease of aggregate subject losses reimbursed by the Federal Government under the Terrorism Risk Insurance Program. + References No definition available. + Details
Name: | sigi_TerrorismRiskInsuranceProgramFederalShareReduction
Namespace Prefix: | sigi_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition The percentage that the insured has to pay for losses under the Terrorism Risk Insurance Program, beginning the next calendar year. + References No definition available. + Details
Name: | sigi_TerrorismRiskInsuranceProgramInsurerRetainer
Namespace Prefix: | sigi_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition The percentage of subject losses, after an insurer deductible, subject to an annual cap, reimbursed by the Federal Government under the Terrorism Risk Insurance Program, beginning the next calendar year. + References No definition available. + Details
Name: | sigi_TerrorismRiskInsuranceProgramReimbursedPercentageMaximum
Namespace Prefix: | sigi_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition The ultimate percentage of subject losses, after an insurer deductible, subject to an annual cap, reimbursed by the Federal Government under the Terrorism Risk Insurance Program by 2020. + References No definition available. + Details
Name: | sigi_TerrorismRiskInsuranceProgramReimbursedPercentageMinimum
Namespace Prefix: | sigi_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition The unexpired portion of premiums ceded on policies in force as of the balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 340 -Section 25 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6485982&loc=d3e22818-158507 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount, after valuation allowance, recoverable under reinsurance contracts including premium paid under reinsurance contracts. Examples include, but are not limited to, settled and unsettled claims, incurred but not reported losses, loss adjustment expense, premium paid, policy benefits and policy reserves. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 20 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6480758&loc=d3e6824-158387 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 310 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6485522&loc=d3e21409-158489 Reference 3: 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),6) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Total of allowances and reserves, the valuation and qualifying accounts that are either netted against the cost of an asset (in order to value it at its carrying value) or that reflect a liability established to represent expected future costs. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 4 -Subparagraph (SX 210.12-09) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e24092-122690 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Details
Name: | us-gaap_CededCreditRiskAxis=sigi_NationalFloodInsuranceProgramMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_CededCreditRiskAxis=sigi_NjUnsatisfiedClaimJudgmentFundMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_CededCreditRiskAxis=sigi_OtherFederalAndStatePoolsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_CededCreditRiskAxis=sigi_TotalFederalAndStatePoolsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_CededCreditRiskAxis=sigi_MunichReGroupMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_CededCreditRiskAxis=sigi_HanoverRuckversicherungsAgMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_CededCreditRiskAxis=sigi_AxisReinsuranceCompanyMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_CededCreditRiskAxis=sigi_SwissReGroupMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_CededCreditRiskAxis=sigi_AllOtherReinsurersMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_CededCreditRiskAxis=sigi_RemainingUnsecuredReinsuranceNetofCollateralMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
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- Details
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Namespace Prefix: |
Data Type: | na
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