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Regulatory Matters (Details) (USD $) In Millions, unless otherwise specified |  |  |  |  |  |  |  |  |  | 3 Months Ended |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
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Dec. 31, 2011 | Dec. 31, 2010 | Jun. 01, 2011 2010 Rate Case [Member] | May 24, 2011 2010 Rate Case [Member] | Dec. 31, 2010 2010 Rate Case [Member] | Nov. 02, 2010 2010 Rate Case [Member] | Dec. 31, 2011 FERC-Approved Wholesale Rates [Member] Customers | Dec. 31, 2011 Non-Affiliated Municipal Customers, Private Utilities [Member] | Sep. 30, 2011 2010 Wisconsin Rate Increase [Member] | Dec. 31, 2010 2010 Wisconsin Rate Increase [Member] | Mar. 31, 2011 DTL Reversal, Revenue Receivable Forgone [Member] | Mar. 31, 2010 DTA Tax Charge, Elimination of Medicare Part D [Member] | Jun. 30, 2011 Income Tax Benefit, MPUC Approval of Deferral of PPACA [Member] | Dec. 31, 2010 Water Rates [Member] 2010 Wisconsin Rate Increase [Member] | Dec. 31, 2010 Natural Gas Rates [Member] 2010 Wisconsin Rate Increase [Member] | Dec. 31, 2010 Electric Rates [Member] 2010 Wisconsin Rate Increase [Member] | Dec. 31, 2011 Deferred Income Tax Charges [Member] | Dec. 31, 2010 Deferred Income Tax Charges [Member] | Dec. 31, 2011 Plant Removal Obligations Costs [Member] | Dec. 31, 2010 Plant Removal Obligations Costs [Member] | Dec. 31, 2011 Other Regulatory Liabilities [Member] | Dec. 31, 2010 Other Regulatory Liabilities [Member] | Dec. 31, 2011 Deferred Fuel Costs [Member] | Dec. 31, 2010 Deferred Fuel Costs [Member] | Dec. 31, 2011 Pension Costs [Member] | Dec. 31, 2010 Pension Costs [Member] | Dec. 31, 2011 Environmental Restoration Costs [Member] | Dec. 31, 2010 Environmental Restoration Costs [Member] | Dec. 31, 2011 Deferred Income Tax Charges [Member] | Dec. 31, 2010 Deferred Income Tax Charges [Member] | Dec. 31, 2011 Asset Retirement Obligation Costs [Member] | Dec. 31, 2010 Asset Retirement Obligation Costs [Member] | Dec. 31, 2011 Medicare Part D Regulatory Asset [Member] | Dec. 31, 2010 Medicare Part D Regulatory Asset [Member] | Dec. 31, 2011 CIP Financial Incentive [Member] | Dec. 31, 2010 CIP Financial Incentive [Member] | Dec. 31, 2011 Other Regulatory Assets [Member] | Dec. 31, 2010 Other Regulatory Assets [Member]
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets, Current | $ 17.5 | $ 20.6 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 17.5 | [1] | $ 20.6 | [1] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets, Noncurrent | 345.9 | 310.2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 292.8 | 257.9 | 0 | 20.5 | 28.6 | 17.3 | 9.8 | 7.8 | 5.0 | 0 | 4.6 | 0.7 | 5.1 | 6.0
Regulatory Assets | 363.4 | 330.8 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Liability, Noncurrent | 43.5 | 43.6 |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 21.9 | 23.4 | 15.0 | 16.9 | 6.6 | 3.3 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Approved Retail Rate Increase |  |  |  | 53.5 |  | 53.5 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Approved Rate of Return on Common Equity |  |  |  |  |  | 10.38% |  |  |  | 10.90% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Approved Percentage of Capital Structure Related to Equity |  |  |  |  |  | 54.29% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
MPUC, Rates, Effective Date |  |  | June 1, 2011 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amount of Rate Differential Authorized for Collection |  |  |  | 3.2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amount of Revenue Receivable Capitalized |  |  |  |  | 20.5 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Tax Adjustments, Settlements, and Unusual Provisions |  |  |  |  |  |  |  |  |  |  | 6.2 | 4.0 | 2.9 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Non-Affiliated Municipal Customers, Municipalities |  |  |  |  |  |  | 16 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Non-Affiliated Municipal Customers, Private Utilities |  |  |  |  |  |  | 1 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Length of Notice Required to Terminate (Years) |  |  |  |  |  |  | 3 | 2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Approved Retail Rate Increase (Percent) |  |  |  |  |  |  |  |  |  | 2.40% |  |  |  | 12.80% | 2.50% | 0.70% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amount Of Estimated Additional Revenue |  |  |  |  |  |  |  |  | $ 2.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
[1] | (a)Current regulatory assets are included in prepayments and other on the consolidated balance sheet.

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- Definition The amount of differential between interim rates and final rates authorized for collection by the state regulatory commission through a rate case proceeding. + References No definition available. + Details
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- Definition The amount of revenue receivable capitalized, as part of rate base, as part of a stipulation and settlement agreement in a rate case. + References No definition available. + Details
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- Definition Percentage of capital structure related to equity approved by the state regulatory commission through a rate case proceeding. + References No definition available. + Details
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- Definition Rate of return on common equity approved by the state regulatory commission through a rate case proceeding. + References No definition available. + Details
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- Definition The entity's approved amount of increase to electric rates, by the state regulatory commission. + References No definition available. + Details
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- Definition The entity's approved rate of increase to existing rates, by the state regulatory commission. + References No definition available. + Details
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- Definition The length of notice required to terminate a contract in years. + References No definition available. + Details
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- Definition Effective date of established rates as approved by the Minnesota Public Utilities Commission (MPUC). + References No definition available. + Details
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- Definition The number of municipalities that are non-affiliated municipal customers. + References No definition available. + Details
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- Details
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- 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 + Details
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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 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 + Details
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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 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 + Details
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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 + Details
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- Definition Reflects the effects of adjustments of previously recorded tax expense, significant settlements of income tax disputes, and unusual tax positions or infrequent actions taken by the entity, including tax assessment reversal, IRS tax settlement and unusual repatriation of foreign earnings. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name FASB Interpretation (FIN) -Number 48 -Paragraph 10 -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. Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name FASB Interpretation (FIN) -Number 48 -Paragraph 10A, 10B, 10C -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 FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (a)(3) -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32718-109319 + Details
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