| **ACCOUNTING POLICIES (Details) (USD $)In Millions**                              | 3 Months Ended | 9 Months Ended |        |
|-----------------------------------------------------------------------------------|----------------|----------------|--------|
||
| **Gas in storage [Abstract]**                                                     |           |           |   |   |   |
| Lower of cost or market valuation charges                                         | $ 5.1          | $ 2.4          | $ 5.9  | $ 7.4 |   |
| **Regulatory Assets [Line Items]**                                                |           |           |   |   |   |
| Regulatory asset - Total                                                          | 295.8          | 287.6          | 295.8  | 287.6 | 277.4 |
| **Regulatory Liabilities [Line Items]**                                           |           |           |   |   |   |
| Regulatory liabilities - Total                                                    | 955.9          | 891.3          | 955.9  | 891.3 | 916.0 |
| **Regulatory assets and liabilities**                                             |           |           |   |   |   |
| Minimum years expected to recover regulatory postretirement asset from ratepayers |           |           | 9      |   |   |
| Maximum years expected to recover regulatory postretirement asset from ratepayers |           |           | 12     |   |   |
| **Revenue Recognition [Abstract]**                                                |           |           |   |   |   |
| Unbilled revenues included in receivables                                         | 34.7           | 29.7           | 34.7   | 29.7 | 144.8 |
| Revenue taxes included in operating expenses                                      | 14.0           | 13.8           | 118.7  | 110.2 |   |
| Regulatory Postretirement Asset [Member]                                          |           |           |   |   |   |
| **Regulatory Assets [Line Items]**                                                |           |           |   |   |   |
| Regulatory assets - current                                                       | 21.1           | 20.6           | 21.1   | 20.6 | 21.1 |
| Regulatory assets - noncurrent                                                    | 181.7          | 183.6          | 181.7  | 183.6 | 193.3 |
| Deferred Gas Costs [Member]                                                       |           |           |   |   |   |
| **Regulatory Assets [Line Items]**                                                |           |           |   |   |   |
| Regulatory assets - current                                                       | 17.7           | 18.1           | 17.7   | 18.1 | 6.5 |
| Regulatory assets - noncurrent                                                    | 0              | 4.3            | 0      | 4.3 | 1.4 |
| Other Regulatory Asset [Member]                                                   |           |           |   |   |   |
| **Regulatory Assets [Line Items]**                                                |           |           |   |   |   |
| Regulatory assets - current                                                       | 9.7            | 7.2            | 9.7    | 7.2 | 7.4 |
| Regulatory assets - noncurrent                                                    | 6.7            | 11.4           | 6.7    | 11.4 | 9.6 |
| Deferred Environmental Costs [Member]                                             |           |           |   |   |   |
| **Regulatory Assets [Line Items]**                                                |           |           |   |   |   |
| Regulatory assets - noncurrent                                                    | 46.6           | 24.8           | 46.6   | 24.8 | 25.0 |
| Unamortized Losses on Reacquired Debt [Member]                                    |           |           |   |   |   |
| **Regulatory Assets [Line Items]**                                                |           |           |   |   |   |
| Regulatory assets - noncurrent                                                    | 12.3           | 13.4           | 12.3   | 13.4 | 13.1 |
| Bad debt rider [Member]                                                           |           |           |   |   |   |
| **Regulatory Assets [Line Items]**                                                |           |           |   |   |   |
| Regulatory assets - current                                                       | 0              | 4.2            | 0      | 4.2 | 0 |
| Bad Debt Rider Member                                                             |           |           |   |   |   |
| **Regulatory Liabilities [Line Items]**                                           |           |           |   |   |   |
| Regulatory liabilities - current                                                  | 26.9           | 0              | 26.9   | 0 | 16.4 |
| Regulatory liabilities - noncurrent                                               | 16.4           | 9.3            | 16.4   | 9.3 | 11.7 |
| Other Regulatory Liability [Member]                                               |           |           |   |   |   |
| **Regulatory Liabilities [Line Items]**                                           |           |           |   |   |   |
| Regulatory liabilities - current                                                  | 2.3            | 11.5           | 2.3    | 11.5 | 7.7 |
| Regulatory liabilities - noncurrent                                               | 1.5            | 0.9            | 1.5    | 0.9 | 0.9 |
| Regulatory Asset Retirement Liability [Member]                                    |           |           |   |   |   |
| **Regulatory Liabilities [Line Items]**                                           |           |           |   |   |   |
| Regulatory liabilities - current                                                  | 17.2           | 14.5           | 17.2   | 14.5 | 17.2 |
| Regulatory liabilities - noncurrent                                               | 878.5          | 836.0          | 878.5  | 836.0 | 843.9 |
| Regulatory Income Tax Liability [Member]                                          |           |           |   |   |   |
| **Regulatory Liabilities [Line Items]**                                           |           |           |   |   |   |
| Regulatory liabilities - noncurrent                                               | $ 13.1         | $ 19.1         | $ 13.1 | $ 19.1 | $ 18.2 |

| X                                                                                                                                               |
|-------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                                                           |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Lower of cost or market valuation charges
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| X                                                                                                                                                                                                                                                                                                       |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The maximum years expected to recover regulatory postretirement asset from ratepayers.
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| X                                                                                                                                                                                                                                                                                                       |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The minimum years expected to recover regulatory postretirement asset from ratepayers.
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|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The amount of excise and sales taxes included in sales and revenues, which are then deducted as a cost of sales. Includes excise taxes, which are applied to specific types of transactions or items (such as gasoline or alcohol); and sales, use and value added taxes, which are applied to a broad class of revenue-producing transactions involving a wide range of goods and services.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 605 -SubTopic 45 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=6408196&loc=d3e61097-111654Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 5 -Section 03 -Paragraph 1Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Emerging Issues Task Force (EITF) -Number 06-3 -Paragraph 4 -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 4: 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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|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The amount for the individual regulatory asset as itemized in a table of regulatory assets as of the end of the period.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 71 -Paragraph 9, 10 -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 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382
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| X                                                                                                                                                                                   |
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|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Carrying amount as of the balance sheet date of capitalized costs of regulated entities that are expected to be recovered through revenue sources within one year or the normal operating cycle, if longer. Such costs are capitalized if they meet both of the following criteria: a. It is probable that future revenue in an amount at least equal to the capitalized cost will result from inclusion of that cost in allowable costs for rate-making purposes. b. Based on available evidence, the future revenue will be provided to permit recovery of the previously incurred cost rather than to provide for expected levels of similar future costs. If the revenue will be provided through an automatic rate-adjustment clause, this criterion requires that the regulator's intent clearly be to permit recovery of the previously incurred cost.
+ References
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|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Carrying amount as of the balance sheet date of capitalized costs of regulated entities that are not expected to be recovered through revenue sources within one year or the normal operating cycle if longer.
+ References
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|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The amount for the individual regulatory liability as itemized in a table of regulatory liabilities as of the end of the period.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 71 -Paragraph 11 -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.
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| 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.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The amount for the individual regulatory current liability as itemized in a table of regulatory current liabilities as of the end of the period.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 71 -Paragraph 11 -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.
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|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The amount for the individual regulatory noncurrent liability as itemized in a table of regulatory noncurrent liabilities as of the end of the period.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 71 -Paragraph 11 -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 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.24) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682
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| - Definition
The amount due for services rendered or products shipped, but not yet billed, recognized in conformity with revenue recognition criteria. This element is distinct from unbilled contracts receivables because this is based on noncontract transactions.
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