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Regulatory Matters - Regulatory Assets (Details) (USD $) In Millions, unless otherwise specified | Mar. 31, 2012 | Dec. 31, 2011
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Regulatory Assets [Line Items] |  |  |  |
Current Regulatory Assets | $ 15.5 |  | $ 17.5 |
Non-Current Regulatory Assets | 339.3 |  | 345.9 |
Regulatory Assets | 354.8 |  | 363.4 |
Deferred Fuel [Member] |  |  |  |
Regulatory Assets [Line Items] |  |  |  |
Current Regulatory Assets | 15.5 | [1] | 17.5 | [1]
Future Benefit Obligations Under Defined Benefit Pension and Other Postretirement Benefit Plans [Member] |  |  |  |
Regulatory Assets [Line Items] |  |  |  |
Non-Current Regulatory Assets | 287.4 |  | 292.8 |
Income Taxes [Member] |  |  |  |
Regulatory Assets [Line Items] |  |  |  |
Non-Current Regulatory Assets | 28.5 |  | 28.6 |
Asset Retirement Obligation [Member] |  |  |  |
Regulatory Assets [Line Items] |  |  |  |
Non-Current Regulatory Assets | 10.3 |  | 9.8 |
PPACA Income Tax Deferral [Member] |  |  |  |
Regulatory Assets [Line Items] |  |  |  |
Non-Current Regulatory Assets | 5.0 |  | 5.0 |
Conservation Improvement Program [Member] |  |  |  |
Regulatory Assets [Line Items] |  |  |  |
Non-Current Regulatory Assets | 2.4 |  | 4.6 |
Other [Member] |  |  |  |
Regulatory Assets [Line Items] |  |  |  |
Non-Current Regulatory Assets | $ 5.7 |  | $ 5.1 |
[1] | Current regulatory assets are included in prepayments and other on the consolidated balance sheet.

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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FASB -Name Statement of Financial Accounting Standard (FAS) -Number 71 -Paragraph 9, 10 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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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