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Reserve for Losses and Loss Expenses (Roll Forward of Gross and Net Environmental Incurred Losses and Loss Expenses and Related Reserves) (Details) (USD $) In Thousands, unless otherwise specified | 12 Months Ended
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Dec. 31, 2011 | Dec. 31, 2010 | Dec. 31, 2009
Reserves for losses and loss expenses at beginning of year, gross | $ 47,291 | $ 50,474 | $ 51,515
Incurred losses and loss expenses, gross | 8,257 | 1,661 | 5,697
Less: losses and loss expenses paid, gross | (9,073) | (4,844) | (6,738)
Reserves for losses and loss expenses at the end of year, gross | 46,475 | 47,291 | 50,474
Reserves for losses and loss expenses at beginning of year, net | 39,449 | 41,602 | 44,093
Incurred losses and loss expenses, net | 8,250 | 2,438 | 3,475
Less: losses and loss expenses paid, net | (9,319) | (4,591) | (5,966)
Reserves for losses and loss expenses at the end of year, net | 38,380 | 39,449 | 41,602
Non Asbestos Related Claims [Member] |  |  |
Reserves for losses and loss expenses at beginning of year, gross | 37,312 | 39,418 | 37,246
Incurred losses and loss expenses, gross | 6,243 | 1,999 | 8,115
Less: losses and loss expenses paid, gross | (5,492) | (4,105) | (5,943)
Reserves for losses and loss expenses at the end of year, gross | 38,063 | 37,312 | 39,418
Reserves for losses and loss expenses at beginning of year, net | 31,282 | 32,358 | 31,124
Incurred losses and loss expenses, net | 6,250 | 2,776 | 6,405
Less: losses and loss expenses paid, net | (5,738) | (3,852) | (5,171)
Reserves for losses and loss expenses at the end of year, net | 31,794 | 31,282 | 32,358
Asbestos Related Claims [Member] |  |  |
Reserves for losses and loss expenses at beginning of year, gross | 9,979 | 11,056 | 14,269
Incurred losses and loss expenses, gross | 2,014 | (338) | (2,418)
Less: losses and loss expenses paid, gross | (3,581) | (739) | (795)
Reserves for losses and loss expenses at the end of year, gross | 8,412 | 9,979 | 11,056
Reserves for losses and loss expenses at beginning of year, net | 8,167 | 9,244 | 12,969
Incurred losses and loss expenses, net | 2,000 | (338) | (2,930)
Less: losses and loss expenses paid, net | (3,581) | (739) | (795)
Reserves for losses and loss expenses at the end of year, net | $ 6,586 | $ 8,167 | $ 9,244

X
- Definition Carrying amount as of the balance sheet date of reserves for the costs of settling insured claims and costs incurred in the claims settlement process attributable to environmental claims, before estimated recoveries from reinsurers. + References No definition available. + Details
Name: | sigi_LiabilityForEnvironmentalClaimsGross
Namespace Prefix: | sigi_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount of environmental claims paid in the current period, before estimated recoveries from reinsurers. + References No definition available. + Details
Name: | sigi_LiabilityForEnvironmentalClaimsGrossClaimsPaid
Namespace Prefix: | sigi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Increase in environmental reserve based on known and estimated costs of settling insured claims and costs incurred in the claims settlement process, before estimated recoveries from reinsurers. + References No definition available. + Details
Name: | sigi_LiabilityForEnvironmentalClaimsGrossIncurredLoss
Namespace Prefix: | sigi_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Carrying amount as of the balance sheet date of reserves for the costs of settling insured claims and costs incurred in the claims settlement process attributable to environmental claims, after estimated recoveries from reinsurers. + References No definition available. + Details
Name: | sigi_LiabilityForEnvironmentalClaimsNet
Namespace Prefix: | sigi_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount of environmental claims paid in the current period, net of reinsurance recoveries. + References No definition available. + Details
Name: | sigi_LiabilityForEnvironmentalClaimsNetClaimsPaid
Namespace Prefix: | sigi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Increase in environmental reserve based on known and estimated costs of settling insured claims and costs incurred in the claims settlement process, net of estimated reinsurance recoveries. + References No definition available. + Details
Name: | sigi_LiabilityForEnvironmentalClaimsNetIncurredLoss
Namespace Prefix: | sigi_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration