Investments (Number of Securities in an Unrealized Unrecognized Loss Position) (Details) (USD $)	9 Months Ended
Sep. 30, 2013 Securities	Dec. 31, 2012 Securities
Schedule of Available-for-sale Securities [Line Items]
Unrealized Unrecognized Losses Period Increase	$ (40,600,000)
Number of securities in an unrealized/unrecognized loss position	502	101
Securities In Unrealized Unrecognized Loss Position For More Than Twelve Months	1,100,000	1,700,000
Unrealized Unrecognized Loss	43,581,000	2,934,000
Eighty Percent to Ninety-Nine Percent [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of securities in an unrealized/unrecognized loss position	501	100
Unrealized Unrecognized Loss	43,295,000	2,701,000
Sixty Percent to Seventy-Nine Percent [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of securities in an unrealized/unrecognized loss position	0	1
Unrealized Unrecognized Loss	0	233,000
Forty Percent to Fifty-Nine Percent [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of securities in an unrealized/unrecognized loss position	1	0
Unrealized Unrecognized Loss	286,000	0
Twenty Percent to Thirty-Nine Percent [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of securities in an unrealized/unrecognized loss position	0	0
Unrealized Unrecognized Loss	0	0
Zero Percent to Nineteen Percent [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of securities in an unrealized/unrecognized loss position	0	0
Unrealized Unrecognized Loss	$ 0	$ 0
10 year U.S. Treasury Note [Member]
Schedule of Available-for-sale Securities [Line Items]
Debt Instruments, Interest Rate, Increase (Decrease)	85

Increase (decrease) in basis points of the stated interest rate on the debt instrument.

No definition available.

Name:	sigi_DebtInstrumentsInterestRateIncreaseDecrease
Namespace Prefix:	sigi_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	duration

- Definition

This item contains disclosure of the number of available-for-sale and held-to-maturity securities determined to be in an unrealized/unrecognized loss position.

+ References

No definition available.

+ Details

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

- Definition

Represents total unrealized and unrecognized losses of all available-for-sale and held-to-maturity securities in an unrealized and unrecognized loss position for more than 12 months as of the end of the period.

+ References

No definition available.

+ Details

Name:	sigi_SecuritiesInUnrealizedUnrecognizedLossPositionForMoreThanTwelveMonthsUnrealizedUnrecognizedLoss
Namespace Prefix:	sigi_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This item represents the sum of (i) the excess of amortized cost over fair value for available-for-sale securities that are in a loss position and (ii) the market fluctuations on held-to-maturity securities from the later of (a) the date a security is designated as held-to-maturity; or (b) the date that an other-than-temporary impairment charge is recognized on a held-to-maturity security for securities that are in an unrecognized loss position.

+ References

No definition available.

+ Details

Name:	sigi_UnrealizedUnrecognizedLoss
Namespace Prefix:	sigi_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Represents the increase or decrease in unrealized/unrecognized losses for available-for-sale and held-to-maturities securities

+ References

No definition available.

+ Details

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

- Details

Name:	us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration