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Property, Plant and Equipment (Narrative) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended |  | 12 Months Ended |  | 12 Months Ended |  | 12 Months Ended |  | 12 Months Ended |  | 12 Months Ended
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Dec. 31, 2013 MW | Dec. 31, 2012 | Dec. 31, 2011 | Dec. 31, 2013 IPL [Member] | Dec. 31, 2012 IPL [Member] | Dec. 31, 2011 IPL [Member] | Dec. 31, 2013 WPL [Member] | Dec. 31, 2012 WPL [Member] | Dec. 31, 2011 WPL [Member] | Dec. 31, 2013 Franklin County Wind Project [Member] | Dec. 31, 2013 Sheboygan Falls Energy Facility [Member] | Dec. 31, 2013 Non-Regulated [Member] Minimum [Member] | Dec. 31, 2013 Non-Regulated [Member] Maximum [Member] | Dec. 31, 2011 Green Lake And Fond Du Lac Counties Wind Site [Member] WPL [Member] | Dec. 31, 2013 Whispering Willow - East Wind Project [Member] | Dec. 31, 2013 Whispering Willow - East Wind Project [Member] IPL [Member] | Dec. 31, 2009 Whispering Willow - East Wind Project [Member] IPL [Member] MW | Dec. 31, 2011 Whispering Willow - East Wind Project [Member] Alliant Energy and IPL [Member] | Dec. 31, 2013 Vestas Wind Turbine Generators [Member] MW | Dec. 31, 2013 Franklin County Wind Project [Member] MW | Dec. 31, 2011 Franklin County Wind Project [Member] MW | Dec. 31, 2013 George Neal Unit 3 [Member] IPL [Member] | Dec. 31, 2013 George Neal Unit 3 [Member] Alliant Energy and IPL [Member] | Dec. 31, 2013 George Neal Unit 4 [Member] IPL [Member] | Dec. 31, 2013 George Neal Unit 4 [Member] Alliant Energy and IPL [Member] | Dec. 31, 2013 Columbia Units 1-2 [Member] WPL [Member] | Dec. 31, 2012 Columbia Units 1-2 [Member] WPL [Member] | Dec. 31, 2011 Columbia Units 1-2 [Member] WPL [Member] | Dec. 31, 2013 Columbia Units 1-2 [Member] Alliant Energy and WPL [Member] | Dec. 31, 2013 IPL (FERC formula) [Member] IPL [Member] | Dec. 31, 2013 Franklin County Iowa Wind Site [Member] IPL [Member] MW | Dec. 31, 2007 Franklin County Iowa Wind Site [Member] IPL [Member] MW | Dec. 31, 2013 Bent Tree - Phase I Wind Project [Member] Whispering Willow - East Wind Project [Member] IPL and WPL [Member] MW | Dec. 31, 2013 Minnesota [Member] Whispering Willow - East Wind Project [Member] IPL [Member] | Dec. 31, 2011 Franklin County Wind Project [Member] IPL [Member] | Dec. 31, 2013 Ottumwa Unit 1 [Member] IPL [Member] | Dec. 31, 2013 Ottumwa Unit 1 [Member] Alliant Energy and IPL [Member] | Dec. 31, 2013 Scenario, Forecast [Member] IPL [Member]
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Wind site capacity (in megawatts) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 200 |  |  |  | 100 |  |  |  |  |  |  |  |  |  | 200 | 500 |  |  |  |  |  |
Other property, plant and equipment | $ 22.3 | $ 21.2 |  | $ 21.2 | $ 19.8 |  | $ 1.1 | $ 1.4 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 13.0 |  |  |  |  |  |  |
Disallowance of project costs |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 8 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Project costs allocated by jurisdiction |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 30 |  |  |  |
Asset impairment charges |  |  |  |  |  |  |  |  |  |  |  |  |  | 5 |  |  |  | 8 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Electric plant | 9,415.7 | 9,070.7 |  | 5,034.9 | 4,815.2 |  | 4,380.8 | 4,255.5 |  |  |  |  |  |  | 7.0 |  |  |  |  |  |  |  |  | 61.0 | 57.0 |  |  |  |  |  |  |  |  |  |  |  |  |
Wind EGU capacity (in megawatts) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 500 | 99 |  |  |  |  |  |  |  |  |  |  |  |  | 401 |  |  |  |  |
Proceeds from sale of wind project assets to affiliate |  |  |  | 0 | 0 | 115.3 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 115.3 |  |  |
Purchase of development rights | 100 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership interest % |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 28.00% |  | 25.695% |  | 46.20% |  |  |  |  |  |  |  |  |  | 48.00% |  |
Allowance for funds used during construction | 30.8 | 21.9 | 12.0 | 21.0 | 8.4 | 5.8 | 9.8 | 13.5 | 6.2 |  |  |  |  |  |  |  |  |  |  |  |  |  | 2.0 |  | 4.0 | 7.2 | 3.9 | 0.2 | 11.0 |  |  |  |  |  |  |  | 10.0 |
George Neal Generating Station Unit 3 emission controls (IPL) | 54.6 | 26.5 |  | 54.6 | 26.5 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 53.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ottumwa Generating Station Unit 1 emission controls (IPL) | 135.1 | 73.7 |  | 135.1 | 73.7 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 125.0 |
Columbia Energy Center Units 1 and 2 emission controls (WPL) | 265.0 | 130.4 |  |  |  |  | 265.0 | 130.4 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 254.0 |  |  |  |  |  |  |  |  |
Expected proceeds from assets sales |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 128
Wholesale power supply agreement termination notice term |  |  |  | 5 years |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Wholesale power supply agreement term |  |  |  | 10 years |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Authorized return on common equity |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 10.97% |  |  |  |  |  |  |  |
Property, plant and equipment, useful life |  |  |  |  |  |  |  |  |  | 30 years | 35 years | 5 years | 30 years |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Non-regulated Generation property, plant and equipment | $ 249.4 | $ 258.6 |  |  |  |  |  |  |  | $ 142.0 | $ 107.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

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