Investments (Number of Securities in an Unrealized Unrecognized Loss Position) (Details) (USD $)	3 Months Ended
Mar. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Unrealized Unrecognized Loss	6,776,000sigi_UnrealizedUnrecognizedLoss	$ 10,596,000sigi_UnrealizedUnrecognizedLoss
Eighty Percent to Ninety-Nine Percent [Member]
Schedule of Available-for-sale Securities [Line Items]
Number Of Securities In An Unrealized/Unrecognized Loss Position	189	350
Unrealized Unrecognized Loss	6,776,000	10,596,000
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	0
Unrealized Unrecognized Loss	0	0
Forty Percent to Fifty-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
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]
Decrease of Yield on 10-year U.S. Treasury Note	25
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Number Of Securities In An Unrealized/Unrecognized Loss Position	28
Unrealized Loss on Securities	3,300,000
Percentage Decline In Fair Value Cost	3.00%

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

No definition available.

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- 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

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- Definition

This item represents the percentage decline in fair value of equity securities compared to cost that have been in a loss position for less than twelve months. This impairment has been determined to be temporary.

+ References

No definition available.

+ Details

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- 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

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- Definition

The increase (decrease), resulting in a loss, in the difference between the fair value and the carrying value, or in the comparative fair values, of securities held at each balance sheet date, that was included in earnings for the period.

+ References

No definition available.

+ Details

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