ACCOUNTING POLICIES (Details) (USD $) In Millions	3 Months Ended	9 Months Ended
Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010	Dec. 31, 2010
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

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Lower of cost or market valuation charges

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The maximum 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

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-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Article 5

-Section 03

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-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.

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

-SubTopic 10

-Section S99

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-Subparagraph (SX 210.5-03.1)

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The amount for the individual regulatory asset as itemized in a table of regulatory assets as of the end of the period.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 71

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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.

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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.

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The amount for the individual regulatory liability as itemized in a table of regulatory liabilities as of the end of the period.

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The amount for the individual regulatory current liability as itemized in a table of regulatory current liabilities as of the end of the period.

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The amount for the individual regulatory noncurrent liability as itemized in a table of regulatory noncurrent liabilities as of the end of the period.

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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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-Name Accounting Standards Codification

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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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