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Regulatory Matters - Regulatory Assets and Liabilities (Details) (USD $) In Millions, unless otherwise specified | Dec. 31, 2012 | Dec. 31, 2011
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Regulatory Assets and Liabilities [Line Items] |  |  |  |
Current Regulatory Asset | $ 22.5 |  | $ 17.5 |
Non-Current Regulatory Asset | 340.3 |  | 345.9 |
Total Regulatory Assets | 362.8 |  | 363.4 |
Non-Current Regulatory Liability | 60.1 |  | 43.5 |
Income Taxes [Member] |  |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |  |
Non-Current Regulatory Liability | 19.5 |  | 21.9 |
Plant Removal Obligations [Member] |  |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |  |
Non-Current Regulatory Liability | 18.1 |  | 15.0 |
Wholesale and Retail Contra AFUDC [Member] |  |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |  |
Non-Current Regulatory Liability | 15.5 |  | 1.5 |
Other [Member] |  |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |  |
Non-Current Regulatory Liability | 7.0 |  | 5.1 |
Deferred Fuel [Member] |  |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |  |
Current Regulatory Asset | 22.5 | [1] | 17.5 | [1]
Future Benefit Obligations Under Defined Benefit Pension and Other Postretirement Benefit Plans [Member] |  |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |  |
Non-Current Regulatory Asset | 260.7 |  | 292.8 |
Income Taxes [Member] |  |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |  |
Non-Current Regulatory Asset | 36.0 |  | 28.6 |
Asset Retirement Obligation [Member] |  |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |  |
Non-Current Regulatory Asset | 12.1 |  | 9.8 |
Cost Recovery Riders [Member] |  |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |  |
Non-Current Regulatory Asset | 18.5 | [2] | 0.7 | [2]
PPACA Income Tax Deferral [Member] |  |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |  |
Non-Current Regulatory Asset | 5.0 |  | 5.0 |
Conservation Improvement Program [Member] |  |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |  |
Non-Current Regulatory Asset | 4.3 |  | 4.6 |
Other [Member] |  |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |  |
Non-Current Regulatory Asset | $ 3.7 |  | $ 4.4 |
[1] | Current regulatory assets are included in prepayments and other on our Consolidated Balance Sheet.
[2] | The increase in cost recovery rider regulatory assets in 2012 is primarily due to revenues related to our Bison Wind Energy Center.

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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 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 Reference 2: 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. + Details
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