| **REGULATORY PROCEEDINGS**            | 6 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |
|---------------------------------------|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
||
| **REGULATORY PROCEEDINGS [Abstract]** | Â                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |
| REGULATORY PROCEEDINGS [Text Block]   | 13.REGULATORY PROCEEDINGSBad debt rider.  In September 2009, Nicor Gas filed for approval of a bad debt rider with the ICC under an Illinois state law which took effect in July 2009.  On February 2, 2010, the ICC issued an order approving the company’s proposed bad debt rider.  This rider provides for recovery from customers of the amount over the benchmark for bad debt expense established in the company’s rate cases.  It also provides for refunds to customers if bad debt expense is below such benchmarks.As a result of the February 2010 order, Nicor Gas recorded in income a net recovery related to 2008 and 2009 of $31.7 million in the first quarter of 2010, all of which has been collected.  The benchmark, against which 2011 and 2010 actual bad debt experience is compared, is approximately $63 million.  The company’s actual 2010 bad debt experience was $35.7 million, resulting in a refund to customers of $27.3 million which is being refunded over a 12-month period beginning June 2011. |

| X                                                                                                                                                        |
|----------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**gas_RegulatoryProceedingsAbstract**Namespace Prefix:**gas**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The entire disclosure for public utilities.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 71 -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_PublicUtilitiesDisclosureTextBlock**Namespace Prefix:**us-gaap**Data Type:**nonnum:textBlockItemType**Balance Type:**na**Period Type:**duration |