| **REGULATORY MATTERS (Details) (USD $)In Millions, unless otherwise specified** | 9 Months Ended  | 12 Months Ended |
|---------------------------------------------------------------------------------|-----------------|-----------------|
||
| **Bad Debt Rider [Abstract]**                                                   |            |            |   |   |
| Description of order issued                                                     | Bad debt rider. |            |   |   |
| Net recovery related to 2008 and 2009 bad debt experience over benchmark        |            | $ 31.7          |   |   |
| Benchmark of the net bad debt rider recovery from customers                     |            |            | 63 | 63 |
| Bad debt experience                                                             |            |            |   | 35.7 |
| Amount of net bad debt rider (recovery from) refund to customers                |            |            |   | $ 27.3 |
| **Substitute Natural Gas: Contract 1 [Abstract]**                               |            |            |   |   |
| Volume Of Substitute Natural Gas (in Bcf): Contract 1                           | 25              |            |   |   |
| Capacity Of Coal Gasification Plant (in Bcf): Contract 1                        | 60              |            |   |   |
| Term (in years): Contract 1                                                     | 10-year         |            |   |   |
| **Substitute Natural Gas: Contract 2 [Abstract]**                               |            |            |   |   |
| Volume of Substitute Natual Gas (in Bcf): Contract 2                            | 20              |            |   |   |
| Term (in years): Contract 2                                                     | 30-year         |            |   |   |

| X                                                                                                                                                                                                                                                                                                                                               |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of recovery from (refund to) customers over (under) the benchmark established in rates cases.
+ Details**Name:**gas_AmountOfRecoveryFromRefundToCustomersOverUnderBenchmarkOfBadDebtExpenseEstablishedInRateCases**Namespace Prefix:**gas**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                       |
|-----------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**gas_BadDebtRider**Namespace Prefix:**gas**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                    |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of 2008 and 2009 bad debt experience over (under) the benchmark established in rates cases.
+ Details**Name:**gas_BedDebtExperienceOverUnderBenchmark**Namespace Prefix:**gas**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                   |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Reflects the benchmark for bad debt expense established in rate cases.
+ Details**Name:**gas_BenchmarkOfNetBadDebtRiderRecoveryFromCustomers**Namespace Prefix:**gas**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Annual capacity (in Bcf) of substitute natural gas plant constructed by counterparty
+ Details**Name:**gas_CapacityOfCoalGasificationPlant**Namespace Prefix:**gas**Data Type:**xbrli:pureItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                            |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
General description of order issued by a regulatory agency.
+ Details**Name:**gas_DescriptionOfOrderIssued**Namespace Prefix:**gas**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                     |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**gas_SubstituteNaturalGas30YearContractAbstract**Namespace Prefix:**gas**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                       |
|---------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**gas_SubstituteNaturalGasAbstract**Namespace Prefix:**gas**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Term (in years) of substitute natural gas contract 1
+ Details**Name:**gas_TermOfContract**Namespace Prefix:**gas**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                            |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Term (in years) of substitute natural gas contract 2
+ Details**Name:**gas_TermOfContract2**Namespace Prefix:**gas**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                      |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Volume (in Bcf) of substitute natural gas that Nicor Gas agreed to purchase annually for a 30-year term
+ Details**Name:**gas_VolumeOfSubstituteNatualGas30YearContract**Namespace Prefix:**gas**Data Type:**xbrli:pureItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                         |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Volume (in Bcf) of substitute natural gas that Nicor Gas agreed to purchase annually for a 10-year term
+ Details**Name:**gas_VolumeOfSubstituteNaturalGas**Namespace Prefix:**gas**Data Type:**xbrli:pureItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of the current period expense charged against operations, the offset which is generally to the allowance for doubtful accounts for the purpose of reducing receivables, including notes receivable, to an amount that approximates their net realizable value (the amount expected to be collected).
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.5) -URI http://asc.fasb.org/extlink&oid=6880815&loc=d3e20235-122688Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 95 -Paragraph 28 -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 SEC -Name Regulation S-X (SX) -Number 210 -Article 5 -Section 03 -Paragraph 5Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6943989&loc=d3e3602-108585
+ Details**Name:**us-gaap_ProvisionForDoubtfulAccounts**Namespace Prefix:**us-gaap**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |