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Utility Rate Cases (Narrative) (Details) (USD $)
In Millions, unless otherwise specified |
3 Months Ended | 9 Months Ended | 1 Months Ended | 3 Months Ended | 9 Months Ended | 1 Months Ended | 9 Months Ended | 1 Months Ended | 9 Months Ended | 3 Months Ended | 9 Months Ended | 1 Months Ended | 9 Months Ended | 1 Months Ended | 9 Months Ended | 3 Months Ended | 9 Months Ended | ||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
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Sep. 30, 2012
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Sep. 30, 2011
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Sep. 30, 2012
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Sep. 30, 2011
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Dec. 31, 2011
2012 Test Year Retail Electric Fuel Related, Final Order [Member]
Cross State Air Pollution Rule [Member]
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Sep. 30, 2012
WPL [Member]
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Sep. 30, 2011
WPL [Member]
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Sep. 30, 2012
WPL [Member]
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Sep. 30, 2011
WPL [Member]
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Sep. 30, 2012
WPL [Member]
WPL's Edgewater Unit 5 Purchase [Member]
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May 31, 2012
WPL [Member]
2013/2014 Test Period Retail Electric And Gas, Request [Member]
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Jun. 30, 2012
WPL [Member]
2013 Test Year Retail Electric Fuel Related, Request [Member]
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Sep. 30, 2012
WPL [Member]
2013 Test Year Retail Electric Fuel Related, Request [Member]
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Oct. 31, 2012
WPL [Member]
2013 Test Year Retail Electric Fuel Related, Oral Decision [Member]
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Dec. 31, 2011
WPL [Member]
2012 Test Year Retail Electric Fuel Related, Final Order [Member]
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Sep. 30, 2012
WPL [Member]
2012 Test Year Retail Electric Fuel Related, Final Order [Member]
|
Sep. 30, 2012
IPL [Member]
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Sep. 30, 2011
IPL [Member]
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Sep. 30, 2012
IPL [Member]
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Sep. 30, 2011
IPL [Member]
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Jun. 30, 2012
IPL [Member]
2011 Test Year Retail Gas IA, Request [Member]
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May 31, 2012
IPL [Member]
2011 Test Year Retail Gas IA, Request [Member]
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Sep. 30, 2012
IPL [Member]
2011 Test Year Retail Gas IA, Request [Member]
Tax Benefit Rider [Member]
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Aug. 31, 2012
IPL [Member]
2011 Test Year Retail Gas IA, Settlement [Member]
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Jul. 31, 2010
IPL [Member]
2009 Test Year Retail Electric MN, Request [Member]
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May 31, 2010
IPL [Member]
2009 Test Year Retail Electric MN, Request [Member]
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Nov. 30, 2011
IPL [Member]
2009 Test Year Retail Electric MN, Final Order [Member]
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Nov. 30, 2011
IPL [Member]
2009 Test Year Retail Electric MN, Final Order [Member]
Higher Base Rates [Member]
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Nov. 30, 2011
IPL [Member]
2009 Test Year Retail Electric MN, Final Order [Member]
Renewable Energy Rider [Member]
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Nov. 30, 2011
IPL [Member]
2009 Test Year Retail Electric MN, Final Order [Member]
IPL Electric Transmission Assets Sale [Member]
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Sep. 30, 2011
IPL [Member]
2009 Test Year Retail Electric MN, Final Order [Member]
Sutherland 4 [Member]
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Sep. 30, 2012
Alliant Energy and IPL [Member]
2011 Test Year Retail Gas IA, Request [Member]
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Sep. 30, 2012
Alliant Energy and IPL [Member]
2011 Test Year Retail Gas IA, Request [Member]
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| Utility Rate Cases [Line Items] | |||||||||||||||||||||||||||||||||
| Reduction in annual rates for its customers, amount | $ 13 | $ 25 | $ 29 | $ 36 | |||||||||||||||||||||||||||||
| Jointly owned utility plant, proportionate ownership share | 25.00% | ||||||||||||||||||||||||||||||||
| Increase in annual rates for its customers, amount | 4 | 15 | 11 | 15 | 11 | 8 | 2 | 1 | |||||||||||||||||||||||||
| Increase in annual rates for its customers, percentage | 6.00% | 22.00% | |||||||||||||||||||||||||||||||
| Increase in interim rates for its customers, amount | 9 | 14 | |||||||||||||||||||||||||||||||
| Increase in interim rates for its customers, percentage | 3.00% | ||||||||||||||||||||||||||||||||
| Interim rate increase for its customers, actual amount | 46.8 | 46.4 | 263.9 | 342.5 | 17.2 | 18.9 | 114.7 | 144.4 | 29.6 | 27.5 | 149.2 | 198.1 | 2.0 | 2.0 | |||||||||||||||||||
| Recovery of previously incurred costs | 2 | ||||||||||||||||||||||||||||||||
| Reduction in annual rates for its customers, percentage | 2.00% | 3.00% | |||||||||||||||||||||||||||||||
| Rate at which fuel costs will be monitored | 2.00% | 2.00% | |||||||||||||||||||||||||||||||
| Deferred compliance costs tolerance band | 0.00% | ||||||||||||||||||||||||||||||||
| Customer refund liability, current | $ 5 | ||||||||||||||||||||||||||||||||