| **EQUITY INVESTMENT INCOME, NET (Details) (USD $)In Millions** | 3 Months Ended | 6 Months Ended |
|----------------------------------------------------------------|----------------|----------------|
||
| **Schedule of Equity Method Investments [Line Items]**         | Â              | Â              | Â | Â |
| Cash distributions from equity investees                       | $ 4.1          | $ 5.4          | $ 8.2 | $ 7.9 |
| Triton [Member]                                                | Â              | Â              | Â | Â |
| **Schedule of Equity Method Investments [Line Items]**         | Â              | Â              | Â | Â |
| Equity investment income, net                                  | $ 3.2          | $ 1.8          | $ 7.4 | $ 3.2 |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This item represents disclosure of the amount of dividends or other distributions received from unconsolidated subsidiaries, certain corporate joint ventures, and certain noncontrolled corporation; these investments are accounted for under the equity method of accounting.
+ Details**Name:**gas_CashDistributionsFromEquityInvestees**Namespace Prefix:**gas**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                            |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This item represents the entity's proportionate share for the period of the net income (loss) of its investment to which the equity method of accounting is applied.
+ Details**Name:**gas_EquityInvestmentIncomeNet**Namespace Prefix:**gas**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                           |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.
+ Details**Name:**us-gaap_ScheduleOfEquityMethodInvestmentsLineItems**Namespace Prefix:**us-gaap**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |