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This element represents the dividends in excess of its continuing subsidiaries' current year earnings for the period, net of tax.

No definition available.

Name:	sigi_DividendsInExcessOfContinuingSubsidiariesCurrentYearEarnings
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This element represents the parent's equity in undistributed income of its continuing subsidiaries for the period, net of tax.

No definition available.

Name:	sigi_EquityInUndistributedIncomeOfContinuingSubsidiariesNetOfTax
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Data Type:	xbrli:monetaryItemType
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The increase or decrease in our long term stock compensation accrued liability

No definition available.

Name:	sigi_IncreaseDecreaseInAccruedLongTermStockCompensation
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Data Type:	xbrli:monetaryItemType
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The cash outflow for debt with subsidiaries initially having maturity due after one year or beyond the normal operating cycle, if longer.

No definition available.

Name:	sigi_PrincipalPaymentsOfBorrowingsFromSubsidiaries
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The sum of adjustments which are added to or deducted from net income or loss, including the portion attributable to noncontrolling interest, to reflect cash provided by or used in operating activities, in accordance with the indirect cash flow method.

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Amount of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Excludes cash and cash equivalents within disposal group and discontinued operation.

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Name:	us-gaap_Cash
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The aggregate net amount of depreciation, amortization, and accretion recognized during an accounting period. As a noncash item, the net amount is added back to net income when calculating cash provided by or used in operations using the indirect method.

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Amount of gain (loss), after tax expense or benefit and not previously recognized, resulting from the sale of a business component.

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Amount of cash inflow from realized tax benefit related to deductible compensation cost reported on the entity's tax return for equity instruments in excess of the compensation cost for those instruments recognized for financial reporting purposes.

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Name:	us-gaap_ExcessTaxBenefitFromShareBasedCompensationFinancingActivities
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The increase (decrease) during the reporting period in the amounts payable to taxing authorities for taxes that are based on the reporting entity's earnings, net of amounts receivable from taxing authorities for refunds of overpayments or recoveries of income taxes, and in deferred and other tax liabilities and assets.

No definition available.

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Balance Type:	debit
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The increase (decrease) during the reporting period in other assets used in operating activities less other operating liabilities used in operating activities not separately disclosed in the statement of cash flows. May include changes in other current assets and liabilities, other noncurrent assets and liabilities, or a combination of other current and noncurrent assets and liabilities.

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The increase (decrease) in cash associated with the entity's continuing operating, investing, and financing activities. While for technical reasons this element has no balance attribute, the default assumption is a debit balance consistent with its label.

No definition available.

Name:	us-gaap_NetCashProvidedByUsedInContinuingOperations
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Data Type:	xbrli:monetaryItemType
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Period Type:	duration

Name:	us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract
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Amount of cash inflow (outflow) of financing activities, excluding discontinued operations. Financing activity cash flows include obtaining resources from owners and providing them with a return on, and a return of, their investment; borrowing money and repaying amounts borrowed, or settling the obligation; and obtaining and paying for other resources obtained from creditors on long-term credit.

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Name:	us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperations
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Period Type:	duration

Name:	us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
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Period Type:	duration

Amount of cash inflow (outflow) of investing activities, excluding discontinued operations. Investing activity cash flows include making and collecting loans and acquiring and disposing of debt or equity instruments and property, plant, and equipment and other productive assets.

Name:	us-gaap_NetCashProvidedByUsedInInvestingActivitiesContinuingOperations
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Name:	us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
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Period Type:	duration

Amount of cash inflow (outflow) from operating activities, excluding discontinued operations. Operating activity cash flows include transactions, adjustments, and changes in value not defined as investing or financing activities.

Name:	us-gaap_NetCashProvidedByUsedInOperatingActivitiesContinuingOperations
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The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

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-Paragraph 1

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-Glossary Other Comprehensive Income

-URI http://asc.fasb.org/extlink&oid=6519514

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-Glossary Net Income

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-Section S99

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-Subparagraph (SX 210.5-03.18)

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-Number 210

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Name:	us-gaap_NetIncomeLoss
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Data Type:	xbrli:monetaryItemType
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The cash outflow to reacquire common stock during the period.

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-Publisher FASB

-Name Accounting Standards Codification

-Glossary Financing Activities

-URI http://asc.fasb.org/extlink&oid=6513228

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Name:	us-gaap_PaymentsForRepurchaseOfCommonStock
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Period Type:	duration

Amount of cash outflow in the form of ordinary dividends to common shareholders of the parent entity.

Name:	us-gaap_PaymentsOfDividendsCommonStock
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
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Period Type:	duration

The cash outflow to acquire debt securities classified as available-for-sale securities, because they are not classified as either held-to-maturity securities or trading securities.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Investing Activities

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

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-Paragraph 13

-Subparagraph (a)

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Name:	us-gaap_PaymentsToAcquireAvailableForSaleSecuritiesDebt
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
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The cash outflow associated with the acquisition of a business, net of the cash acquired from the purchase.

Name:	us-gaap_PaymentsToAcquireBusinessesNetOfCashAcquired
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
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The cash outflow for securities or other assets acquired, which qualify for treatment as an investing activity and are to be liquidated, if necessary, within the current operating cycle. Includes cash flows from securities classified as trading securities that were acquired for reasons other than sale in the short-term.

Name:	us-gaap_PaymentsToAcquireShortTermInvestments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

This element represents the cash inflow during the period from the sale of a component of the entity.

Name:	us-gaap_ProceedsFromDivestitureOfBusinessesNetOfCashDivested
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
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Period Type:	duration

The cash inflow from a debt initially having maturity due after one year or beyond the operating cycle, if longer.

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-Publisher FASB

-Name Accounting Standards Codification

-Glossary Financing Activities

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Name:	us-gaap_ProceedsFromIssuanceOfLongTermDebt
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Data Type:	xbrli:monetaryItemType
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Period Type:	duration

The total cash inflow associated with the amount received from holders to acquire the entity's shares under incentive and share awards, including stock option exercises. This item inherently excludes any excess tax benefit, which the entity may have realized and reported separately.

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-Glossary Financing Activities

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The cash inflow associated with maturities (principal being due), prepayments and calls (requests of early payments) on securities not classified as either held-to-maturity securities or trading securities which are classified as available-for-sale securities.

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-Glossary Investing Activities

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The cash inflow associated with the maturity, prepayments and calls (requests for early payments) of debt securities designated as held-to-maturity.

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-Publisher FASB

-Name Accounting Standards Codification

-Glossary Investing Activities

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-Topic 230

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Name:	us-gaap_ProceedsFromMaturitiesPrepaymentsAndCallsOfHeldToMaturitySecurities
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Data Type:	xbrli:monetaryItemType
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The cash inflow from sales of all investments, including securities and other assets, having ready marketability and intended by management to be liquidated, if necessary, within the current operating cycle. Includes cash flows from securities classified as trading securities that were acquired for reasons other than sale in the short-term.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

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-Topic 230

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-Glossary Investing Activities

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Name:	us-gaap_ProceedsFromSaleOfShortTermInvestments
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Data Type:	xbrli:monetaryItemType
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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.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

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-Paragraph 1

-Subparagraph (SX 210.7-04.3(a))

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-Paragraph 1

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-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph 3

-Subparagraph a

-Article 7

Name:	us-gaap_RealizedInvestmentGainsLosses
Namespace Prefix:	us-gaap_
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The cash outflow for debt initially having maturity due after one year or beyond the normal operating cycle, if longer.

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-Glossary Financing Activities

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Name:	us-gaap_RepaymentsOfLongTermDebt
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The aggregate amount of noncash, equity-based employee remuneration. This may include the value of stock or unit options, amortization of restricted stock or units, and adjustment for officers' compensation. As noncash, this element is an add back when calculating net cash generated by operating activities using the indirect method.

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Name:	us-gaap_ShareBasedCompensation
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