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Subsequent Events Subsequent Events (Details) - Subsequent Event [Member] $ in Millions	3 Months Ended
	Dec. 31, 2016 USD ($)
Subsequent Event [Line Items]
Flood Claims Handling Fees	$ 1
Minimum [Member]
Subsequent Event [Line Items]
Estimated Catastrophe Losses	10
Maximum [Member]
Subsequent Event [Line Items]
Estimated Catastrophe Losses	$ 14

X

- Definition

Estimated catastrophe losses as a result of severe weather conditions.

+ References

No definition available.

+ Details

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Namespace Prefix:	sigi_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Definition

Commissions received from the NFIP in handling flood claims.

+ References

No definition available.

+ Details

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Namespace Prefix:	sigi_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- Definition

Detail information of subsequent event by type. User is expected to use existing line items from elsewhere in the taxonomy as the primary line items for this disclosure, which is further associated with dimension and member elements pertaining to a subsequent event.

+ References

No definition available.

+ Details

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Period Type:	duration

X

- Details

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Namespace Prefix:
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Balance Type:
Period Type:

X

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

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Namespace Prefix:
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