Investments (Number of Securities in an Unrealized Unrecognized Loss Position) (Details) (USD $)	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013 Securities
Number of securities in an unrealized/unrecognized loss position	350	557
Unrealized Unrecognized Loss	10,596,000sigi_UnrealizedUnrecognizedLoss	$ 52,011,000sigi_UnrealizedUnrecognizedLoss
Securities In Unrealized Unrecognized Loss Position For More Than Twelve Months	8,200,000	2,900,000
Number Of Securities With Unrealized Unrecognized Losses Over 12 Months That Experienced Rating Downgrade	1
Percentage Decline in Fair Value Percentage	2.00%	6.00%
Eighty Percent to Ninety-Nine Percent [Member]
Number of securities in an unrealized/unrecognized loss position	350	556
Unrealized Unrecognized Loss	10,596,000	51,835,000
Sixty Percent to Seventy-Nine Percent [Member]
Number of securities in an unrealized/unrecognized loss position	0	1
Unrealized Unrecognized Loss	0	176,000
Forty Percent to Fifty-Nine Percent [Member]
Number of securities in an unrealized/unrecognized loss position	0	0
Unrealized Unrecognized Loss	0	0
Twenty Percent to Thirty-Nine Percent [Member]
Number of securities in an unrealized/unrecognized loss position	0	0
Unrealized Unrecognized Loss	0	0
Zero Percent to Nineteen Percent [Member]
Number of securities in an unrealized/unrecognized loss position	0	0
Unrealized Unrecognized Loss	0	0
10 Year U.S. Treasury Note [Member]
Increase of Yield on Note	86
Securities With Rating Downgrades [Member]
Percentage Decline in Fair Value Percentage	13.00%	9.00%
Securities Rating Downgrade [Member]
Unrealized Unrecognized Loss	1,100,000

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

Number of securities that have been in a loss position for more than twelve months that have experienced a rating downgrade during the period.

+ References

No definition available.

+ Details

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- Definition

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

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No definition available.

+ Details

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- 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

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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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