| **POSTRETIREMENT BENEFITS (Policies)**   | 6 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |
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| **POSTRETIREMENT BENEFITS [Abstract]**   | Â                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           |
| Postretirement benefit accounting policy | The company’s postretirement benefit costs have historically been considered in rate proceedings in the period they are accrued.  As a regulated utility, Nicor Gas expects continued rate recovery of the eligible costs of its defined benefit postretirement plans and, accordingly, associated changes in the plan’s funded status have been deferred as a regulatory asset or liability until recognized in net income, instead of being recorded in accumulated other comprehensive income.  However, to the extent Nicor Gas employees perform services for non-regulated affiliates and to the extent such employees are eligible to participate in these plans, the affiliates are charged for the cost of these benefits and the changes in the funded status relating to these employees are recorded in accumulated other comprehensive income. |

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| - Definition
Disclosure of accounting policy for pension and other postretirement benefit plans. May include descriptions of regulatory treatment for: rate recovery of eligible costs, and, accordingly, associated changes in the plan's funded status which may be deferred as a regulatory asset or liability
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| - Details**Name:**gas_PostretirementBenefitsAbstract**Namespace Prefix:**gas**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |