| **BUSINESS SEGMENT INFORMATION (Details) (USD $)In Millions** | 3 Months Ended | 9 Months Ended |
|---------------------------------------------------------------|----------------|----------------|
||
| **Operating revenues [Abstract]**                             |           |           |   |   |
| External customers                                            | $ 348.4        | $ 352.5        | $ 1,862.4 | $ 1,971.0 |
| Intersegment                                                  | 0              | 0              | 0 | 0 |
| Total operating revenues                                      | 348.4          | 352.5          | 1,862.4 | 1,971.0 |
| Operating income (loss)                                       | 18.3           | 30.4           | 119.3 | 169.8 |
| Natural weather hedge cost (benefit)                          | 0.3            | (0.1)          | 2.6 | 0.3 |
| Merger related costs incurred and expensed                    | 0.4            |           | 2.8 |   |
| Gas Distribution [Member]                                     |           |           |   |   |
| **Operating revenues [Abstract]**                             |           |           |   |   |
| External customers                                            | 235.4          | 229.0          | 1,485.7 | 1,571.3 |
| Intersegment                                                  | 4.5            | 4.5            | 28.9 | 30.5 |
| Total operating revenues                                      | 239.9          | 233.5          | 1,514.6 | 1,601.8 |
| Operating income (loss)                                       | 21.3           | 20.9           | 115.1 | 141.5 |
| Shipping [Member]                                             |           |           |   |   |
| **Operating revenues [Abstract]**                             |           |           |   |   |
| External customers                                            | 78.4           | 83.9           | 239.1 | 253.4 |
| Intersegment                                                  | 0              | 0              | 0 | 0 |
| Total operating revenues                                      | 78.4           | 83.9           | 239.1 | 253.4 |
| Operating income (loss)                                       | (3.7)          | 3.6            | (5.2) | 7.3 |
| Wholesale Marketing [Member]                                  |           |           |   |   |
| **Operating revenues [Abstract]**                             |           |           |   |   |
| External customers                                            | 1.6            | 7.1            | 12.8 | 20.5 |
| Intersegment                                                  | 0.7            | 1.1            | 8.4 | 9.3 |
| Total operating revenues                                      | 2.3            | 8.2            | 21.2 | 29.8 |
| Operating income (loss)                                       | (4.7)          | 0.7            | (0.1) | 8.1 |
| Other [Member]                                                |           |           |   |   |
| **Operating revenues [Abstract]**                             |           |           |   |   |
| External customers                                            | 33.0           | 32.5           | 124.8 | 125.8 |
| Intersegment                                                  | 0.5            | 0.5            | 1.3 | 1.6 |
| Total operating revenues                                      | 33.5           | 33.0           | 126.1 | 127.4 |
| Operating income (loss)                                       | 6.4            | 6.4            | 15.7 | 14.8 |
| Corporate and eliminations [Member]                           |           |           |   |   |
| **Operating revenues [Abstract]**                             |           |           |   |   |
| External customers                                            | 0              | 0              | 0 | 0 |
| Intersegment                                                  | (5.7)          | (6.1)          | (38.6) | (41.4) |
| Total operating revenues                                      | (5.7)          | (6.1)          | (38.6) | (41.4) |
| Operating income (loss)                                       | $ (1.0)        | $ (1.2)        | $ (6.2) | $ (1.9) |

| 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                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
A cost or benefit attributable to variances from normal weather conditions associated with utility-bill management contracts. This cost is recorded primarily at the corporate level and partially offsets the weather variances in the gas distribution segment.
+ Details**Name:**gas_NaturalWeatherHedge**Namespace Prefix:**gas**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The net result for the period of deducting operating expenses from operating revenues.
+ Details**Name:**us-gaap_OperatingIncomeLoss**Namespace Prefix:**us-gaap**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                  |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The total amount of operating revenues recognized during the period.
+ Details**Name:**us-gaap_RegulatedAndUnregulatedOperatingRevenue**Namespace Prefix:**us-gaap**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Aggregate revenue recognized during the period (derived from goods sold, services rendered, insurance premiums, or other activities that constitute an entity's earning process). For financial services companies, also includes investment and interest income, and sales and trading gains.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 5 -Section 03 -Paragraph 1Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.1) -URI http://asc.fasb.org/extlink&oid=6880815&loc=d3e20235-122688
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of revenue from transactions with other operating segments of the same entity.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 131 -Paragraph 27 -Subparagraph b -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_RevenuesFromTransactionsWithOtherOperatingSegmentsOfSameEntity**Namespace Prefix:**us-gaap**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                             |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_SegmentReportingInformationRevenueAbstract**Namespace Prefix:**us-gaap**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |