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Subsequent events Subsequent event (Narrative) (Details) (USD $) In Millions, unless otherwise specified		1 Months Ended
	Dec. 31, 2013	Jan. 31, 2014 Maximum [Member]	Jan. 31, 2014 Minimum [Member]
Estimated catastrophe losses		$ 32	$ 28
State Footprint	22

X

- Definition

Estimated catastrophe losses as a result of severe weather conditions.

+ References

No definition available.

+ Details

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

X

- Definition

Represents the number of states in our footprint.

+ References

No definition available.

+ Details

Name:	sigi_StateFootprint
Namespace Prefix:	sigi_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant