| **CONTINGENCIES (Details) (USD $)In Millions, unless otherwise specified**                                                                     |     | 12 Months Ended |      |        | 6 Months Ended |     |
|------------------------------------------------------------------------------------------------------------------------------------------------|-----|-----------------|------|--------|----------------|-----|
||
| **PBR Plan Claims [Abstract]**                                                                                                                 | Â   | Â               | Â    | Â      | Â              | Â   | Â | Â | Â |
| Initial PBR reserve                                                                                                                            | Â   | Â               | Â    | $ 24.8 | Â              | Â   | Â | Â | Â |
| Loss contingency included in PBR reserve                                                                                                       | Â   | Â               | 4.1  | Â      | Â              | Â   | Â | Â | Â |
| 2004 adjustment to previously recorded PBR reserve                                                                                             | Â   | 1.8             | Â    | Â      | Â              | Â   | Â | Â | Â |
| Recorded PBR reserve                                                                                                                           | Â   | Â               | 26.6 | Â      | Â              | Â   | Â | Â | Â |
| Unrecognized 2002 PBR claimed receivable                                                                                                       | Â   | Â               | 26.9 | Â      | Â              | Â   | Â | Â | Â |
| Amount of sanctions sought per the February 3, 2003 CUB motion                                                                                 | Â   | Â               | Â    | Â      | Â              | Â   | 27.0 | Â | Â |
| Unrecognized recovery sought by Nicor Gas                                                                                                      | Â   | Â               | 6.0  | Â      | Â              | Â   | Â | Â | Â |
| Unrecognized amount of interest included in recovery amount sought by Nicor Gas                                                                | Â   | Â               | 1.6  | Â      | Â              | Â   | Â | Â | Â |
| Amount of customer refunds requested by parties                                                                                                | Â   | Â               | Â    | Â      | 109            | 255 | 286 | Â | Â |
| **Litigation Relating to Proposed Merger [Abstract]**                                                                                          | Â   | Â               | Â    | Â      | Â              | Â   | Â | Â | Â |
| Proposed settlement amount                                                                                                                     | 0.7 | Â               | Â    | Â      | Â              | Â   | Â | Â | Â |
| **Mercury Loss Contingency Estimate [Abstract]**                                                                                               | Â   | Â               | Â    | Â      | Â              | Â   | Â | Â | Â |
| Reserve recorded for environmental matters                                                                                                     | Â   | Â               | Â    | Â      | Â              | Â   | Â | 0.5 | Â |
| **Manufactured Gas Plant Sites Loss Contingency Estimate [Abstract]**                                                                          | Â   | Â               | Â    | Â      | Â              | Â   | Â | Â | Â |
| Percent of cleanup costs allocated to company for certain sites (in hundredths)                                                                | Â   | Â               | Â    | Â      | Â              | Â   | Â | Â | 51.73% |
| Number of sites for which company has 51.73% of clean-up responsibility                                                                        | Â   | Â               | Â    | Â      | Â              | Â   | Â | Â | 24 |
| Number of sites with no cleanup responsibility                                                                                                 | Â   | Â               | Â    | Â      | Â              | Â   | Â | Â | 14 |
| Percent of costs allocated to Company for general remediation program costs that do not relate exclusively to particular sites (in hundredths) | Â   | Â               | Â    | Â      | Â              | Â   | Â | Â | 50.00% |
| Reserve Recorded                                                                                                                               | Â   | Â               | Â    | Â      | Â              | Â   | Â | Â | $ 52.9 |

| X                                                                                                                                                                                                                                                      |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
2004 adjustment to previously recorded reserve or the PBR Plan.
+ Details**Name:**gas_AdjustmentToPreviouslyRecordedPbrReserve**Namespace Prefix:**gas**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                       |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The amount of sanctions sought against Nicor Gas from a plaintiff related to PBR plan.
+ Details**Name:**gas_AmountOfSanctionsSoughtPerCubMotion**Namespace Prefix:**gas**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of reserve related to the PBR Plan reflected in financial statements prior to the 2004 adjustment.
+ Details**Name:**gas_InitialPbrReserve**Namespace Prefix:**gas**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                     |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**gas_LitigationRelatingToProposedMergerAbstract**Namespace Prefix:**gas**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                       |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Loss contingency included in reserve for the PBR Plan.
+ Details**Name:**gas_LossContingencyIncludedInPbrReserve**Namespace Prefix:**gas**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                   |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**gas_ManufacturedGasPlantSitesLossContingencyEstimateAbstract**Namespace Prefix:**gas**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                 |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**gas_MercuryLossContingencyEstimateAbstract**Namespace Prefix:**gas**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                      |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number of sites for which Nicor Gas is allocated 51.73 percent of clean-up costs by agreement between Nicor Gas and Commonwealth Edison Company.
+ Details**Name:**gas_NumberOfSites**Namespace Prefix:**gas**Data Type:**xbrli:integerItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                         |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number of sites for which Nicor Gas is allocated no percentage of cleanup costs pursuant to agreement between Nicor Gas and Commonwealth Edison Company.
+ Details**Name:**gas_NumberOfSitesWithNoCleanupResponsibility**Namespace Prefix:**gas**Data Type:**xbrli:integerItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**gas_PbrPlanClaimsAbstract**Namespace Prefix:**gas**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                    |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Percent of cleanup costs allocated to company for certain manufactured gas plant sites by agreement between Nicor Gas and Commonwealth Edison Company.
+ Details**Name:**gas_PercentOfCleanupCostsAllocatedToCompany**Namespace Prefix:**gas**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Percent of general remediation program costs allocated to Nicor Gas that do not relate exclusively to particular sites pursuant to agreement between Nicor Gas and Commonwealth Edison Company.
+ Details**Name:**gas_PercentOfGeneralRemediationProgramCostsThatDoNotRelateExclusivelyToParticularSites**Namespace Prefix:**gas**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                              |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Proposed settlement amount of litigation relating to the proposed merger between Nicor and AGL Resources.
+ Details**Name:**gas_ProposedSettlementAmount**Namespace Prefix:**gas**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                              |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Total amount of loss contingency reserve related to the PBR plan as of the balance sheet date.
+ Details**Name:**gas_RecordedPbrReserve**Namespace Prefix:**gas**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                  |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Total costs accrued as of the balance sheet date for Manufactured Gas Plant Sites loss contingencies.
+ Details**Name:**gas_ReserveRecorded**Namespace Prefix:**gas**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                      |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount claimed due to the company for the 2002 PBR plan year which has not been recognized in the financial statements due to uncertainties surrounding the PBR plan.
+ Details**Name:**gas_Unrecognized2002pbrClaimedReceivable**Namespace Prefix:**gas**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                               |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of interest included in recovery amount sought related to the PBR Plan.
+ Details**Name:**gas_UnrecognizedAmountOfInterestIncludedInRecoveryAmountSoughtByNicorGas**Namespace Prefix:**gas**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                      |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of reimbursement sought by Nicor Gas related to the PBR Plan after it's 2007 direct testimony.
+ Details**Name:**gas_UnrecognizedRecoverySoughtByNicorGas**Namespace Prefix:**gas**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Total costs accrued as of the balance sheet date for environmental loss contingencies.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 30 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=6393242&loc=d3e13207-110859Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 25 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6395460&loc=d3e13647-108346Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Statement of Position (SOP) -Number 96-1 -Paragraph 161 -Subparagraph a -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 5 -Paragraph 9 -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 450 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6952336&loc=d3e14326-108349
+ Details**Name:**us-gaap_AccrualForEnvironmentalLossContingencies**Namespace Prefix:**us-gaap**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The value (monetary amount) of the award the plaintiff seeks in the legal matter.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6952336&loc=d3e14326-108349Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 50 -Paragraph 4 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6952336&loc=d3e14435-108349Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 5 -Paragraph 9 -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 Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 50 -Paragraph 9 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6952336&loc=d3e14557-108349Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 5 -Paragraph 11 -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 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 5 -Paragraph 10 -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 7: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 5 -Paragraph 12 -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.
+ Details**Name:**us-gaap_LossContingencyDamagesSoughtValue**Namespace Prefix:**us-gaap**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |