| **PROPOSED MERGER WITH AGL RESOURCES (Details) (USD $)In Millions, except Share data**                                                                         | 3 Months Ended | 6 Months Ended |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------|----------------|----------------|
||
| **PROPOSED MERGER WITH AGL RESOURCES [Abstract]**                                                                                                              | Â              | Â              |
| Cash consideration into which each outstanding share of Nicor's stock will be converted in business merger (dollars per share)                                 | $ 21.20        | $ 21.20        |
| Number of shares of AGL Resources' common stock to be received for each share of Nicor common stock in accordance with the merger agreement (shares per share) | 0.8382         | 0.8382         |
| Merger related costs incurred and expensed                                                                                                                     | $ 0.9          | $ 2.4          |

| X                                                                                                                                                                                                                                                                                                                                                                                                      |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This element represents the proposed merger-date cash consideration to be received by holders for each outstanding, noncancelled and non-dissenting share of common stock of Nicor.
+ Details**Name:**gas_BusinessCombinationProposedCashConsiderationToBeReceivedPerShareCommonStock**Namespace Prefix:**gas**Data Type:**num:perShareItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This element represents the proposed merger-date number of common shares of AGL Resources equity consideration to be received by holders for each outstanding, noncancelled and non-dissenting share of common stock of Nicor.
+ Details**Name:**gas_BusinessCombinationProposedEquityConsiderationToBeReceivedPerShareCommonStock**Namespace Prefix:**gas**Data Type:**xbrli:sharesItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                   |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of direct costs of the business combination including legal, accounting, and other costs incurred to consummate the business acquisition.
+ Details**Name:**gas_MergerRelatedCostsIncurredAndExpensed**Namespace Prefix:**gas**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                 |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**gas_ProposedMergerWithAglResourcesAbstract**Namespace Prefix:**gas**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |