Commitments and Contingencies (Future Minimum Rental Commitments) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014
2015- Capital Leases	$ 3.1
2016- Capital Leases	1.8
2017- Capital Leases	0.8
2018- Capital Leases	0
2019- Capital Leases	0
After 2019- Capital Leases	0
Total minimum payment required- Capital Leases	5.7
2015- Operating Leases	9.1
2016- Operating Leases	6.9
2017- Operating Leases	5.9
2018- Operating Leases	4.7
2019- Operating Leases	3.7
After 2019- Operating Leases	8.9
Total minimum payment required- Operating Leases	39.2
2015- Total Leases	12.2sigi_TotalLeasesDueCurrentYear
2016- Total Leases	8.7sigi_TotalLeasesDueInTwoYears
2017- Total Leases	6.7sigi_TotalLeasesDueInThreeYears
2018- Total Leases	4.7sigi_TotalLeasesDueInFourYears
2019- Total Leases	3.7sigi_TotalLeasesDueInFiveYears
After 2019- Total Leases	8.9sigi_TotalLeasesDueThereafter
Total minimum payment required	$ 44.9sigi_TotalMinimumPaymentRequired

Amount of minimum lease payments maturing in the next fiscal year following the latest fiscal year for all leases.

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Amount of minimum lease payments maturing in the fifth fiscal year following the latest fiscal year for all leases.

+ References

No definition available.

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Amount of minimum lease payments maturing in the forth fiscal year following the latest fiscal year for all leases.

+ References

No definition available.

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Amount of minimum lease payments maturing in the third fiscal year following the latest fiscal year for all leases.

+ References

No definition available.

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Amount of minimum lease payments maturing in the second fiscal year following the latest fiscal year for all leases

+ References

No definition available.

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Amount of minimum lease payments maturing after the fifth fiscal year following the latest fiscal year for all leases.

+ References

No definition available.

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Amount of minimum lease payments for all leases.

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No definition available.

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Amount of minimum lease payments for capital leases.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 840

-SubTopic 30

-Section 50

-Paragraph 1

-Subparagraph (b)

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Amount of minimum lease payments for capital leases due in the next fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of minimum lease payments for capital leases due in the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of minimum lease payments for capital leases due in the fourth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of minimum lease payments for capital leases due in the third fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of minimum lease payments for capital leases due in the second fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of minimum lease payments for capital leases due after the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

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Amount of required minimum rental payments for leases having an initial or remaining non-cancelable letter-terms in excess of one year.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 840

-SubTopic 20

-Section 50

-Paragraph 2

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=6453985&loc=d3e41502-112717

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Amount of required minimum rental payments for operating leases having an initial or remaining non-cancelable lease term in excess of one year due in the next fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of required minimum rental payments for operating leases having an initial or remaining non-cancelable lease term in excess of one year due in the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of required minimum rental payments for operating leases having an initial or remaining non-cancelable lease term in excess of one year due in the fourth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of required minimum rental payments for operating leases having an initial or remaining non-cancelable lease term in excess of one year due in the third fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of required minimum rental payments for operating leases having an initial or remaining non-cancelable lease term in excess of one year due in the second fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of required minimum rental payments for operating leases having an initial or remaining non-cancelable lease term in excess of one year due after the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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