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Retirement Plans (Quantitative Disclosures of Retirement Income Plan's Invested Assets that are Measured at Fair Value on a Recurring Basis) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Alternative Investment Securities Sold		3
Fair Value [Member] | Fair Value, Measurements, Recurring [Member]
Total assets	$ 182,774	$ 171,980
Fair Value [Member] | Mutual Funds [Member] | Fair Value, Measurements, Recurring [Member]
Total assets	137,610	116,436
Fair Value [Member] | Equity Securities [Member] | Fair Value, Measurements, Recurring [Member]
Total assets	19,944	20,140
Fair Value [Member] | Alternative Investments [Member] | Fair Value, Measurements, Recurring [Member]
Total assets	17,016	18,958
Fair Value [Member] | Short-term Investments [Member] | Fair Value, Measurements, Recurring [Member]
Total assets	7,225	15,692
Fair Value [Member] | Deposit Administration Contracts [Member] | Fair Value, Measurements, Recurring [Member]
Total assets	979	754
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair Value, Measurements, Recurring [Member]
Total assets	164,779	152,268
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Mutual Funds [Member] | Fair Value, Measurements, Recurring [Member]
Total assets	137,610	116,436
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Equity Securities [Member] | Fair Value, Measurements, Recurring [Member]
Total assets	19,944	20,140
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Alternative Investments [Member] | Fair Value, Measurements, Recurring [Member]
Total assets	0	0
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Short-term Investments [Member] | Fair Value, Measurements, Recurring [Member]
Total assets	7,225	15,692
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Deposit Administration Contracts [Member] | Fair Value, Measurements, Recurring [Member]
Total assets	0	0
Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member]
Total assets	2,815	2,533
Significant Other Observable Inputs (Level 2) [Member] | Mutual Funds [Member] | Fair Value, Measurements, Recurring [Member]
Total assets	0	0
Significant Other Observable Inputs (Level 2) [Member] | Equity Securities [Member] | Fair Value, Measurements, Recurring [Member]
Total assets	0	0
Significant Other Observable Inputs (Level 2) [Member] | Alternative Investments [Member] | Fair Value, Measurements, Recurring [Member]
Total assets	1,836	1,779
Significant Other Observable Inputs (Level 2) [Member] | Short-term Investments [Member] | Fair Value, Measurements, Recurring [Member]
Total assets	0	0
Significant Other Observable Inputs (Level 2) [Member] | Deposit Administration Contracts [Member] | Fair Value, Measurements, Recurring [Member]
Total assets	979	754
Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member]
Total assets	15,180	17,179
Significant Unobservable Inputs (Level 3) [Member] | Mutual Funds [Member] | Fair Value, Measurements, Recurring [Member]
Total assets	0	0
Significant Unobservable Inputs (Level 3) [Member] | Equity Securities [Member] | Fair Value, Measurements, Recurring [Member]
Total assets	0	0
Significant Unobservable Inputs (Level 3) [Member] | Alternative Investments [Member]
Total assets	15,180	17,179	18,764
Significant Unobservable Inputs (Level 3) [Member] | Alternative Investments [Member] | Fair Value, Measurements, Recurring [Member]
Total assets	15,180	17,179
Significant Unobservable Inputs (Level 3) [Member] | Short-term Investments [Member] | Fair Value, Measurements, Recurring [Member]
Total assets	0	0
Significant Unobservable Inputs (Level 3) [Member] | Deposit Administration Contracts [Member] | Fair Value, Measurements, Recurring [Member]
Total assets	0	0
International Securities [Member] | Fair Value [Member] | Mutual Funds [Member] | Fair Value, Measurements, Recurring [Member]
Total assets	13,205	14,277
International Securities [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Mutual Funds [Member] | Fair Value, Measurements, Recurring [Member]
Total assets	13,205	14,277
International Securities [Member] | Significant Other Observable Inputs (Level 2) [Member] | Mutual Funds [Member] | Fair Value, Measurements, Recurring [Member]
Total assets	0	0
International Securities [Member] | Significant Unobservable Inputs (Level 3) [Member] | Mutual Funds [Member] | Fair Value, Measurements, Recurring [Member]
Total assets	0	0
Large Capitalization Securities [Member] | Fair Value [Member] | Mutual Funds [Member] | Fair Value, Measurements, Recurring [Member]
Total assets	22,200	20,580
Large Capitalization Securities [Member] | Fair Value [Member] | Equity Securities [Member] | Fair Value, Measurements, Recurring [Member]
Total assets	11,618	11,509
Large Capitalization Securities [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Mutual Funds [Member] | Fair Value, Measurements, Recurring [Member]
Total assets	22,200	20,580
Large Capitalization Securities [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Equity Securities [Member] | Fair Value, Measurements, Recurring [Member]
Total assets	11,618	11,509
Large Capitalization Securities [Member] | Significant Other Observable Inputs (Level 2) [Member] | Mutual Funds [Member] | Fair Value, Measurements, Recurring [Member]
Total assets	0	0
Large Capitalization Securities [Member] | Significant Other Observable Inputs (Level 2) [Member] | Equity Securities [Member] | Fair Value, Measurements, Recurring [Member]
Total assets	0	0
Large Capitalization Securities [Member] | Significant Unobservable Inputs (Level 3) [Member] | Mutual Funds [Member] | Fair Value, Measurements, Recurring [Member]
Total assets	0	0
Large Capitalization Securities [Member] | Significant Unobservable Inputs (Level 3) [Member] | Equity Securities [Member] | Fair Value, Measurements, Recurring [Member]
Total assets	0	0
Small And Mid Capitalization Securities [Member] | Fair Value [Member] | Mutual Funds [Member] | Fair Value, Measurements, Recurring [Member]
Total assets	6,750	6,785
Small And Mid Capitalization Securities [Member] | Fair Value [Member] | Equity Securities [Member] | Fair Value, Measurements, Recurring [Member]
Total assets	8,326	8,631
Small And Mid Capitalization Securities [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Mutual Funds [Member] | Fair Value, Measurements, Recurring [Member]
Total assets	6,750	6,785
Small And Mid Capitalization Securities [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Equity Securities [Member] | Fair Value, Measurements, Recurring [Member]
Total assets	8,326	8,631
Small And Mid Capitalization Securities [Member] | Significant Other Observable Inputs (Level 2) [Member] | Mutual Funds [Member] | Fair Value, Measurements, Recurring [Member]
Total assets	0	0
Small And Mid Capitalization Securities [Member] | Significant Other Observable Inputs (Level 2) [Member] | Equity Securities [Member] | Fair Value, Measurements, Recurring [Member]
Total assets	0	0
Small And Mid Capitalization Securities [Member] | Significant Unobservable Inputs (Level 3) [Member] | Mutual Funds [Member] | Fair Value, Measurements, Recurring [Member]
Total assets	0	0
Small And Mid Capitalization Securities [Member] | Significant Unobservable Inputs (Level 3) [Member] | Equity Securities [Member] | Fair Value, Measurements, Recurring [Member]
Total assets	0	0
Liability Driven Debt Investments [Member] | Fair Value [Member] | Mutual Funds [Member] | Fair Value, Measurements, Recurring [Member]
Total assets	36,881	27,726
Liability Driven Debt Investments [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Mutual Funds [Member] | Fair Value, Measurements, Recurring [Member]
Total assets	36,881	27,726
Liability Driven Debt Investments [Member] | Significant Other Observable Inputs (Level 2) [Member] | Mutual Funds [Member] | Fair Value, Measurements, Recurring [Member]
Total assets	0	0
Liability Driven Debt Investments [Member] | Significant Unobservable Inputs (Level 3) [Member] | Mutual Funds [Member] | Fair Value, Measurements, Recurring [Member]
Total assets	0	0
Domestic Core Debt Securities [Member] | Fair Value [Member] | Mutual Funds [Member] | Fair Value, Measurements, Recurring [Member]
Total assets	32,930	26,991
Domestic Core Debt Securities [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Mutual Funds [Member] | Fair Value, Measurements, Recurring [Member]
Total assets	32,930	26,991
Domestic Core Debt Securities [Member] | Significant Other Observable Inputs (Level 2) [Member] | Mutual Funds [Member] | Fair Value, Measurements, Recurring [Member]
Total assets	0	0
Domestic Core Debt Securities [Member] | Significant Unobservable Inputs (Level 3) [Member] | Mutual Funds [Member] | Fair Value, Measurements, Recurring [Member]
Total assets	0	0
Global Emerging Market Debt Securities [Member] | Fair Value [Member] | Mutual Funds [Member] | Fair Value, Measurements, Recurring [Member]
Total assets	25,644	20,077
Global Emerging Market Debt Securities [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Mutual Funds [Member] | Fair Value, Measurements, Recurring [Member]
Total assets	25,644	20,077
Global Emerging Market Debt Securities [Member] | Significant Other Observable Inputs (Level 2) [Member] | Mutual Funds [Member] | Fair Value, Measurements, Recurring [Member]
Total assets	0	0
Global Emerging Market Debt Securities [Member] | Significant Unobservable Inputs (Level 3) [Member] | Mutual Funds [Member] | Fair Value, Measurements, Recurring [Member]
Total assets	0	0
Equity Long Short Hedge [Member] | Fair Value [Member] | Alternative Investments [Member] | Fair Value, Measurements, Recurring [Member]
Total assets	1,836	1,779
Equity Long Short Hedge [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Alternative Investments [Member] | Fair Value, Measurements, Recurring [Member]
Total assets	0	0
Equity Long Short Hedge [Member] | Significant Other Observable Inputs (Level 2) [Member] | Alternative Investments [Member] | Fair Value, Measurements, Recurring [Member]
Total assets	1,836	1,779
Equity Long Short Hedge [Member] | Significant Unobservable Inputs (Level 3) [Member] | Alternative Investments [Member] | Fair Value, Measurements, Recurring [Member]
Total assets	0	0
Private Equity [Member] | Fair Value [Member] | Alternative Investments [Member] | Fair Value, Measurements, Recurring [Member]
Total assets	12,586	14,492
Private Equity [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Alternative Investments [Member] | Fair Value, Measurements, Recurring [Member]
Total assets	0	0
Private Equity [Member] | Significant Other Observable Inputs (Level 2) [Member] | Alternative Investments [Member] | Fair Value, Measurements, Recurring [Member]
Total assets	0	0
Private Equity [Member] | Significant Unobservable Inputs (Level 3) [Member] | Alternative Investments [Member] | Fair Value, Measurements, Recurring [Member]
Total assets	12,586	14,492
Real Estate Strategy [Member] | Fair Value [Member] | Alternative Investments [Member] | Fair Value, Measurements, Recurring [Member]
Total assets	2,594	2,687
Real Estate Strategy [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Alternative Investments [Member] | Fair Value, Measurements, Recurring [Member]
Total assets	0	0
Real Estate Strategy [Member] | Significant Other Observable Inputs (Level 2) [Member] | Alternative Investments [Member] | Fair Value, Measurements, Recurring [Member]
Total assets	0	0
Real Estate Strategy [Member] | Significant Unobservable Inputs (Level 3) [Member] | Alternative Investments [Member] | Fair Value, Measurements, Recurring [Member]
Total assets	$ 2,594	$ 2,687

X

- Definition

This item represents the number of alternative investments that were sold in the secondary market.

+ References

No definition available.

+ Details

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

X

- Definition

Assets, usually stocks, bonds, and other investments, that have been segregated and restricted (usually in a trust) to provide benefits, at their fair value as of the measurement date. Plan assets include amounts contributed by the employer (and by employees for a contributory plan) and amounts earned from investing the contributions, less benefits paid. If a plan has liabilities other than for benefits, those non-benefit obligations may be considered as reductions of plan assets.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 30

-Section 35

-Paragraph 50

-URI http://asc.fasb.org/extlink&oid=6867990&loc=d3e12355-114930



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 132R

-Paragraph 5

-Subparagraph d(iv)(b)(i)

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.



Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 132R

-Paragraph 5

-Subparagraph b

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.



Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 87

-Paragraph 49

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.



Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=6962843&loc=d3e1928-114920

+ Details

Name:	us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant