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Property, Plant and Equipment (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2011 | Dec. 31, 2010
Property, Plant and Equipment [Line Items] |  |
Property, Plant and Equipment - Net | $ 1,982.7 | $ 1,805.6
Asset Retirement Obligation [Roll Forward] |  |
Beginning Obligation | 50.3 | 44.6
Accretion Expense | 6.4 | 2.9
Additional Liabilities Incurred | 0.3 | 2.8
Ending Obligation | 57.0 | 50.3
Regulated Utiity [Member] |  |
Property, Plant and Equipment [Line Items] |  |
Segment Property, Plant and Equipment | 2,794.8 | 2,649.2
Construction Work in Progress | 155.0 | 86.6
Accumulated Depreciation | (1,024.6) | (975.8)
Segment Property, Plant and Equipment - Net | 1,925.2 | 1,760.0
Estimated Useful Life - Generation | 4 to 35 years |
Estimated Useful Life - Transmission | 42 to 61 years |
Estimated Useful Life - Distribution | 14 to 65 years |
Unregulated Non-Rate Base Energy Operations and Other Plant [Member] |  |
Property, Plant and Equipment [Line Items] |  |
Segment Property, Plant and Equipment | 106.4 | 88.4
Construction Work in Progress | 2.3 | 4.5
Accumulated Depreciation | (51.4) | (48.0)
Segment Property, Plant and Equipment - Net | 57.3 | 44.9
Other Plant - Net | $ 0.2 | $ 0.7
Estimated Useful Life - Non-Rate Base Operations | 3 to 61 years |
Estimated Useful Life - Other Plant | 5 to 25 years |

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- Definition The carrying amount of a liability for an asset retirement obligation. An asset retirement obligation is a legal obligation associated with the disposal or retirement of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 143 -Paragraph 3, 10, 22 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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 410 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Asset Retirement Obligation -URI http://asc.fasb.org/extlink&oid=6505190 + Details
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- Definition Tangible assets that are held by an entity for use in the production or supply of goods and services, for rental to others, or for administrative purposes and that are expected to provide economic benefit for more than one year; net of accumulated depreciation. Examples include land, buildings, machinery and equipment, and other types of furniture and equipment including, but not limited to, office equipment, furniture and fixtures, and computer equipment and software. + 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 210.5-02.13) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 7 -Section 03 -Paragraph 8 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 13 -Subparagraph a -Article 5 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 12 -Paragraph 5 -Subparagraph b, c -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 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 360 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6391035&loc=d3e2868-110229 + Details
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