v2.4.0.6

Summary Of Significant Accounting Policies (Narrative) (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended | 9 Months Ended |  |  | 3 Months Ended | 9 Months Ended |  |  | 3 Months Ended | 9 Months Ended |  |  | 9 Months Ended | 1 Months Ended | 9 Months Ended |  | 9 Months Ended | 3 Months Ended | 12 Months Ended | 9 Months Ended | 3 Months Ended | 9 Months Ended | 1 Months Ended | 3 Months Ended | 9 Months Ended | 12 Months Ended | 3 Months Ended | 9 Months Ended |
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Sep. 30, 2012 | Sep. 30, 2011 | Sep. 30, 2012 | Sep. 30, 2011 | Dec. 31, 2011 | Dec. 31, 2010 | Sep. 30, 2012 WPL [Member] | Sep. 30, 2011 WPL [Member] | Sep. 30, 2012 WPL [Member] | Sep. 30, 2011 WPL [Member] | Dec. 31, 2011 WPL [Member] | Dec. 31, 2010 WPL [Member] | Sep. 30, 2012 IPL [Member] | Sep. 30, 2011 IPL [Member] | Sep. 30, 2012 IPL [Member] | Sep. 30, 2011 IPL [Member] | Dec. 31, 2011 IPL [Member] | Dec. 31, 2010 IPL [Member] | Sep. 30, 2012 George Neal Units 3 And 4 [Member] Alliant Energy and IPL [Member] | Sep. 30, 2012 Ottumwa Unit 1 [Member] Alliant Energy and IPL [Member] | Aug. 31, 2011 Sutherland 4 [Member] Alliant Energy and IPL [Member] | Sep. 30, 2011 Sutherland 4 [Member] Alliant Energy and IPL [Member] | Sep. 30, 2011 IPL Electric Transmission Assets Sale [Member] Alliant Energy and IPL [Member] | Sep. 30, 2012 WPL's Edgewater Unit 5 Purchase [Member] WPL [Member] | Sep. 30, 2012 WPL 2013/2014 Test Period Retail Electric And Gas Final Order [Member] Alliant Energy and WPL [Member] | Sep. 30, 2012 Edgewater Unit 5 Emission Controls [Member] Alliant Energy and WPL [Member] | Sep. 30, 2012 Columbia Units 1 And 2 [Member] Alliant Energy and WPL [Member] | Sep. 30, 2012 Vestas Wind Turbine Generators [Member] MW | Sep. 30, 2012 Franklin County Wind Project [Member] MW | Mar. 31, 2011 Green Lake and Fond du Lac Counties Wind Site [Member] WPL [Member] | Dec. 31, 2009 Green Lake and Fond du Lac Counties Wind Site [Member] WPL [Member] MW | Sep. 30, 2012 Whispering Willow - East Wind Project [Member] IPL [Member] | Sep. 30, 2011 Whispering Willow - East Wind Project [Member] Alliant Energy and IPL [Member] | Sep. 30, 2012 Tax Benefit Rider [Member] | Sep. 30, 2012 Tax Benefit Rider [Member] IPL [Member] | Sep. 30, 2011 Tax Benefit Rider [Member] IPL [Member] | Sep. 30, 2012 Tax Benefit Rider [Member] IPL [Member] | Sep. 30, 2011 Tax Benefit Rider [Member] IPL [Member] | Sep. 30, 2012 Franklin County Wind Project [Member] | Sep. 30, 2012 Franklin County Wind Project [Member] Bent Tree - Phase I Wind Project [Member] IPL and WPL [Member] MW | Apr. 30, 2012 Corporate Headquarters [Member] | Jun. 30, 2011 Franklin County Wind Project [Member] IPL [Member] | Sep. 30, 2012 Scenario, Forecast [Member] WPL [Member] | Sep. 30, 2012 Scenario, Forecast [Member] IPL [Member] | Sep. 30, 2012 Scenario, Forecast [Member] Electric Generation Equipment [Member] WPL [Member] | Sep. 30, 2012 Scenario, Forecast [Member] Electric Distribution [Member] WPL [Member] | Sep. 30, 2012 Scenario, Forecast [Member] Gas Transmission [Member] WPL [Member] | Dec. 31, 2007 SO2 emission allowances [Member] IPL [Member] | Sep. 30, 2011 Cross State Air Pollution Rule [Member] Alliant Energy and IPL [Member] | Sep. 30, 2012 IPL's tax benefit rider [Member] Alliant Energy and IPL [Member] | Sep. 30, 2012 Energy conservation cost recovery [Member] Alliant Energy and WPL [Member] | Sep. 30, 2012 George Neal Unit 3 [Member] IPL [Member] | Sep. 30, 2012 Columbia Units 1 And 2 [Member] WPL [Member] | Sep. 30, 2012 George Neal Unit 4 [Member] IPL [Member] | Sep. 30, 2012 Ottumwa Unit 1 [Member] IPL [Member]
Public Utility, Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Minimum future commitments |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 355 | $ 605 |  |  |  | $ 34 | $ 34 |  |  |  |  |  |
Regulatory assets | 1,518.4 |  | 1,518.4 |  | 1,495.0 |  | 367.4 |  | 367.4 |  | 377.7 |  | 1,151.0 |  | 1,151.0 |  | 1,117.3 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 30.0 |  |  |  |  |  |
Amounts not probable of recovery |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 4 |  |  |  |  |  |
Electric production fuel and energy purchases | 221.6 | 215.3 | 550.4 | 590.0 |  |  | 104.9 | 105.4 | 277.6 | 295.9 |  |  | 116.7 | 109.9 | 272.8 | 294.1 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2.0 |  |  |  |  |  |
Utilities operating expense, maintenance and operations | 144.7 | 147.1 | 432.6 | 476.6 |  |  | 58.3 | 62.3 | 175.5 | 194.6 |  |  | 86.4 | 84.8 | 257.1 | 282.0 |  |  |  |  |  |  | 5.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2.0 |  |  |  |  |  |
Recovery of direct costs |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase (decrease) in regulatory assets |  |  | 73.5 | 191.1 |  |  |  |  | 15.8 | (10.9) |  |  |  |  | 57.7 | 202.0 |  |  |  |  |  | 2.0 |  |  | 5.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Credit to utility other operation and maintenance |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2 |  |  | 5 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory liabilities |  |  | 5.0 | 158.5 |  |  |  |  | 32.5 | 13.4 |  |  |  |  | (27.5) | 145.1 |  |  |  |  |  |  | 5.0 |  |  |  |  |  |  |  |  |  |  | 63.0 | 23.0 | 20.0 | 63.0 | 44.0 |  |  |  |  |  |  |  |  |  |  |  | 22.0 | 15.0 |  |  |  |
Amount of impairment to carrying amount of regulatory assets |  | 7 |  | 7 |  |  |  | 5 |  | 5 |  |  |  | 2 |  | 2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Jointly owned utility plant, proportionate ownership share |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 25.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 28.00% | 46.20% | 25.695% | 48.00%
Capitalized expenditures |  |  | 412.7 | 480.2 |  |  |  |  | 218.1 | 247.9 |  |  |  |  | 194.6 | 232.3 |  |  | 53.0 | 52.0 |  |  |  |  |  | 116.0 | 90.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Allowance for funds used during construction | 5.8 | 2.8 | 14.4 | 8.6 |  |  | 3.7 | 1.5 | 9.1 | 4.3 |  |  | 2.1 | 1.3 | 5.3 | 4.3 |  |  | 1.0 | 1.0 |  |  |  |  |  | 8.0 | 2.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Electric capacity of wind project (in megawatts) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 500 | 100 |  |  |  |  |  |  |  |  |  |  | 400 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from sale of wind project assets to affiliate |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 0 | 115.3 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 115.3 |  |  |  |  |  |  |  |  |  |  |  |  |
Public utilities, property, plant and equipment, amount of indirect disallowance of costs of recently completed plants |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 8 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Project costs allocated by jurisdiction |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 30 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Asset impairment loss |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 5 |  |  | 8 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Purchase of development rights (in megawatts) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 100 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public utilities, property, plant and equipment, disclosure of composite depreciation rate for plants in service |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 3.40% | 2.70% | 2.50% |  |  |  |  |  |  |  |
Non-regulated Generation capitalized expenditures, less accumulated depreciation | 326.5 |  | 326.5 |  | 270.6 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 198.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest costs, capitalized during period |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 8 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Asset retirement obligations | 97.2 | 90.6 | 97.2 | 90.6 | 91.1 | 75.9 | 42.9 | 34.5 | 42.9 | 34.5 | 34.9 | 32.3 | 45.9 | 56.1 | 45.9 | 56.1 | 56.2 | 43.6 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 8.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Alliant Energy Corporate Services, Inc. and non-regulated businesses capitalized expenditures |  |  | $ 106.3 | $ 46.1 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 48.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |

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- Definition The cash outflow for purchases of and capital improvements on property, plant and equipment (capital expenditures), software, and other intangible assets for Alliant Energy Corporate Services, Inc. and non-regulated business. + References No definition available. + Details
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- Definition Discloses the amount of any reduction to the carrying amounts of regulated assets that result from impairment (for example, rate actions of a regulator). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 71 -Paragraph 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 35 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6066304&loc=d3e44026-110379 + Details
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- Definition The increase (decrease) during the reporting period in the value of other assets that are created when regulatory agencies permits public utilities to defer costs to the balance sheet. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 95 -Paragraph 28 -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 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6943989&loc=d3e3602-108585 + Details
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- Definition The increase (decrease) during the reporting period in regulated liabilities. Regulated liabilities are created when regulatory agency permits an entity to defer revenues to the balance sheet. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 95 -Paragraph 28 -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 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6943989&loc=d3e3602-108585 + Details
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- Definition The amount of interest that was capitalized during the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 34 -Paragraph 21 -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 Accounting Standards Codification -Topic 835 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6450988&loc=d3e26243-108391 + Details
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- Definition Total increase in earnings in the period representing the cost of equity (based on assumed rate of return) and/or borrowed funds (based on interest rate) used to finance construction of regulated assets, which is expected to be recovered through rate adjustments. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6943989&loc=d3e3602-108585 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 360 -Section 25 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6500269&loc=d3e45485-110386 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 71 -Paragraph 15 -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 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44278-110382 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 90 -Paragraph 8 -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 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 92 -Paragraph 8, 9, 12 -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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- Definition The amount of indirect disallowance of costs of a recently completed plant. When it becomes probable that part of the cost of a recently completed plant will be disallowed for rate-making purposes and a reasonable estimate of the amount of the disallowance can be made, the estimated amount of the probable disallowance is deducted from the reported cost of the plant and recognized as a loss. If part of the cost is explicitly, but indirectly, disallowed (for example, by an explicit disallowance of return on investment on a portion of the plant), an equivalent amount of cost is deducted from the reported cost of the plant and recognized as a loss. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 90 -Paragraph 7, 60 -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 360 -Section 35 -Paragraph 12 -URI http://asc.fasb.org/extlink&oid=6500292&loc=d3e45799-110387 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 360 -Section 35 -Paragraph 13 -URI http://asc.fasb.org/extlink&oid=6500292&loc=d3e45819-110387 + Details
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- Definition Discloses the overall composite depreciation rate for pubic utility plants in service for each year an income statement is presented. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 360 -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 10.C) -URI http://asc.fasb.org/extlink&oid=6500719&loc=d3e659983-123033 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 210.5-02.13(a)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition Return of or reimbursements received in relation to direct costs and expenses previously paid or incurred. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.2) -URI http://asc.fasb.org/extlink&oid=6880815&loc=d3e20235-122688 + 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 Discloses the amount of operating expense for the period for routine plant maintenance, repairs and operations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.2(b)) -URI http://asc.fasb.org/extlink&oid=6880815&loc=d3e20235-122688 + Details
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