Derivative Instruments (Notional Amounts Of Derivative Instruments) (Details) (Commodity [Member])	Sep. 30, 2012 MWh
Electricity (MWhs) [Member] | Alliant Energy [Member]
Notional Amount of Derivatives [Line Items]
Notional Unit Amount Of Derivatives (in MWhs/MWs/Dths/Tons)	8,134,000
Electricity (MWhs) [Member] | IPL [Member]
Notional Amount of Derivatives [Line Items]
Notional Unit Amount Of Derivatives (in MWhs/MWs/Dths/Tons)	3,130,000
Electricity (MWhs) [Member] | WPL [Member]
Notional Amount of Derivatives [Line Items]
Notional Unit Amount Of Derivatives (in MWhs/MWs/Dths/Tons)	5,004,000
FTRs (MWs) [Member] | Alliant Energy [Member]
Notional Amount of Derivatives [Line Items]
Notional Unit Amount Of Derivatives (in MWhs/MWs/Dths/Tons)	44,000
FTRs (MWs) [Member] | IPL [Member]
Notional Amount of Derivatives [Line Items]
Notional Unit Amount Of Derivatives (in MWhs/MWs/Dths/Tons)	23,000
FTRs (MWs) [Member] | WPL [Member]
Notional Amount of Derivatives [Line Items]
Notional Unit Amount Of Derivatives (in MWhs/MWs/Dths/Tons)	21,000
Natural Gas (Dths) [Member] | Alliant Energy [Member]
Notional Amount of Derivatives [Line Items]
Notional Unit Amount Of Derivatives (in MWhs/MWs/Dths/Tons)	77,379,000
Natural Gas (Dths) [Member] | IPL [Member]
Notional Amount of Derivatives [Line Items]
Notional Unit Amount Of Derivatives (in MWhs/MWs/Dths/Tons)	54,818,000
Natural Gas (Dths) [Member] | WPL [Member]
Notional Amount of Derivatives [Line Items]
Notional Unit Amount Of Derivatives (in MWhs/MWs/Dths/Tons)	22,561,000
Coal (Tons) [Member] | Alliant Energy [Member]
Notional Amount of Derivatives [Line Items]
Notional Unit Amount Of Derivatives (in MWhs/MWs/Dths/Tons)	2,498,000
Coal (Tons) [Member] | WPL [Member]
Notional Amount of Derivatives [Line Items]
Notional Unit Amount Of Derivatives (in MWhs/MWs/Dths/Tons)	2,498,000
2012 [Member] | Electricity (MWhs) [Member] | Alliant Energy [Member]
Notional Amount of Derivatives [Line Items]
Notional Unit Amount Of Derivatives (in MWhs/MWs/Dths/Tons)	1,280,000
2012 [Member] | Electricity (MWhs) [Member] | IPL [Member]
Notional Amount of Derivatives [Line Items]
Notional Unit Amount Of Derivatives (in MWhs/MWs/Dths/Tons)	786,000
2012 [Member] | Electricity (MWhs) [Member] | WPL [Member]
Notional Amount of Derivatives [Line Items]
Notional Unit Amount Of Derivatives (in MWhs/MWs/Dths/Tons)	494,000
2012 [Member] | FTRs (MWs) [Member] | Alliant Energy [Member]
Notional Amount of Derivatives [Line Items]
Notional Unit Amount Of Derivatives (in MWhs/MWs/Dths/Tons)	16,000
2012 [Member] | FTRs (MWs) [Member] | IPL [Member]
Notional Amount of Derivatives [Line Items]
Notional Unit Amount Of Derivatives (in MWhs/MWs/Dths/Tons)	8,000
2012 [Member] | FTRs (MWs) [Member] | WPL [Member]
Notional Amount of Derivatives [Line Items]
Notional Unit Amount Of Derivatives (in MWhs/MWs/Dths/Tons)	8,000
2012 [Member] | Natural Gas (Dths) [Member] | Alliant Energy [Member]
Notional Amount of Derivatives [Line Items]
Notional Unit Amount Of Derivatives (in MWhs/MWs/Dths/Tons)	26,070,000
2012 [Member] | Natural Gas (Dths) [Member] | IPL [Member]
Notional Amount of Derivatives [Line Items]
Notional Unit Amount Of Derivatives (in MWhs/MWs/Dths/Tons)	19,274,000
2012 [Member] | Natural Gas (Dths) [Member] | WPL [Member]
Notional Amount of Derivatives [Line Items]
Notional Unit Amount Of Derivatives (in MWhs/MWs/Dths/Tons)	6,796,000
2012 [Member] | Coal (Tons) [Member] | Alliant Energy [Member]
Notional Amount of Derivatives [Line Items]
Notional Unit Amount Of Derivatives (in MWhs/MWs/Dths/Tons)	0
2012 [Member] | Coal (Tons) [Member] | WPL [Member]
Notional Amount of Derivatives [Line Items]
Notional Unit Amount Of Derivatives (in MWhs/MWs/Dths/Tons)	0
2013 [Member] | Electricity (MWhs) [Member] | Alliant Energy [Member]
Notional Amount of Derivatives [Line Items]
Notional Unit Amount Of Derivatives (in MWhs/MWs/Dths/Tons)	3,860,000
2013 [Member] | Electricity (MWhs) [Member] | IPL [Member]
Notional Amount of Derivatives [Line Items]
Notional Unit Amount Of Derivatives (in MWhs/MWs/Dths/Tons)	1,978,000
2013 [Member] | Electricity (MWhs) [Member] | WPL [Member]
Notional Amount of Derivatives [Line Items]
Notional Unit Amount Of Derivatives (in MWhs/MWs/Dths/Tons)	1,882,000
2013 [Member] | FTRs (MWs) [Member] | Alliant Energy [Member]
Notional Amount of Derivatives [Line Items]
Notional Unit Amount Of Derivatives (in MWhs/MWs/Dths/Tons)	28,000
2013 [Member] | FTRs (MWs) [Member] | IPL [Member]
Notional Amount of Derivatives [Line Items]
Notional Unit Amount Of Derivatives (in MWhs/MWs/Dths/Tons)	15,000
2013 [Member] | FTRs (MWs) [Member] | WPL [Member]
Notional Amount of Derivatives [Line Items]
Notional Unit Amount Of Derivatives (in MWhs/MWs/Dths/Tons)	13,000
2013 [Member] | Natural Gas (Dths) [Member] | Alliant Energy [Member]
Notional Amount of Derivatives [Line Items]
Notional Unit Amount Of Derivatives (in MWhs/MWs/Dths/Tons)	44,899,000
2013 [Member] | Natural Gas (Dths) [Member] | IPL [Member]
Notional Amount of Derivatives [Line Items]
Notional Unit Amount Of Derivatives (in MWhs/MWs/Dths/Tons)	32,309,000
2013 [Member] | Natural Gas (Dths) [Member] | WPL [Member]
Notional Amount of Derivatives [Line Items]
Notional Unit Amount Of Derivatives (in MWhs/MWs/Dths/Tons)	12,590,000
2013 [Member] | Coal (Tons) [Member] | Alliant Energy [Member]
Notional Amount of Derivatives [Line Items]
Notional Unit Amount Of Derivatives (in MWhs/MWs/Dths/Tons)	956,000
2013 [Member] | Coal (Tons) [Member] | WPL [Member]
Notional Amount of Derivatives [Line Items]
Notional Unit Amount Of Derivatives (in MWhs/MWs/Dths/Tons)	956,000
2014 [Member] | Electricity (MWhs) [Member] | Alliant Energy [Member]
Notional Amount of Derivatives [Line Items]
Notional Unit Amount Of Derivatives (in MWhs/MWs/Dths/Tons)	2,118,000
2014 [Member] | Electricity (MWhs) [Member] | IPL [Member]
Notional Amount of Derivatives [Line Items]
Notional Unit Amount Of Derivatives (in MWhs/MWs/Dths/Tons)	366,000
2014 [Member] | Electricity (MWhs) [Member] | WPL [Member]
Notional Amount of Derivatives [Line Items]
Notional Unit Amount Of Derivatives (in MWhs/MWs/Dths/Tons)	1,752,000
2014 [Member] | FTRs (MWs) [Member] | Alliant Energy [Member]
Notional Amount of Derivatives [Line Items]
Notional Unit Amount Of Derivatives (in MWhs/MWs/Dths/Tons)	0
2014 [Member] | FTRs (MWs) [Member] | IPL [Member]
Notional Amount of Derivatives [Line Items]
Notional Unit Amount Of Derivatives (in MWhs/MWs/Dths/Tons)	0
2014 [Member] | FTRs (MWs) [Member] | WPL [Member]
Notional Amount of Derivatives [Line Items]
Notional Unit Amount Of Derivatives (in MWhs/MWs/Dths/Tons)	0
2014 [Member] | Natural Gas (Dths) [Member] | Alliant Energy [Member]
Notional Amount of Derivatives [Line Items]
Notional Unit Amount Of Derivatives (in MWhs/MWs/Dths/Tons)	6,410,000
2014 [Member] | Natural Gas (Dths) [Member] | IPL [Member]
Notional Amount of Derivatives [Line Items]
Notional Unit Amount Of Derivatives (in MWhs/MWs/Dths/Tons)	3,235,000
2014 [Member] | Natural Gas (Dths) [Member] | WPL [Member]
Notional Amount of Derivatives [Line Items]
Notional Unit Amount Of Derivatives (in MWhs/MWs/Dths/Tons)	3,175,000
2014 [Member] | Coal (Tons) [Member] | Alliant Energy [Member]
Notional Amount of Derivatives [Line Items]
Notional Unit Amount Of Derivatives (in MWhs/MWs/Dths/Tons)	981,000
2014 [Member] | Coal (Tons) [Member] | WPL [Member]
Notional Amount of Derivatives [Line Items]
Notional Unit Amount Of Derivatives (in MWhs/MWs/Dths/Tons)	981,000
2015 [Member] | Electricity (MWhs) [Member] | Alliant Energy [Member]
Notional Amount of Derivatives [Line Items]
Notional Unit Amount Of Derivatives (in MWhs/MWs/Dths/Tons)	876,000
2015 [Member] | Electricity (MWhs) [Member] | IPL [Member]
Notional Amount of Derivatives [Line Items]
Notional Unit Amount Of Derivatives (in MWhs/MWs/Dths/Tons)	0
2015 [Member] | Electricity (MWhs) [Member] | WPL [Member]
Notional Amount of Derivatives [Line Items]
Notional Unit Amount Of Derivatives (in MWhs/MWs/Dths/Tons)	876,000
2015 [Member] | FTRs (MWs) [Member] | Alliant Energy [Member]
Notional Amount of Derivatives [Line Items]
Notional Unit Amount Of Derivatives (in MWhs/MWs/Dths/Tons)	0
2015 [Member] | FTRs (MWs) [Member] | IPL [Member]
Notional Amount of Derivatives [Line Items]
Notional Unit Amount Of Derivatives (in MWhs/MWs/Dths/Tons)	0
2015 [Member] | FTRs (MWs) [Member] | WPL [Member]
Notional Amount of Derivatives [Line Items]
Notional Unit Amount Of Derivatives (in MWhs/MWs/Dths/Tons)	0
2015 [Member] | Natural Gas (Dths) [Member] | Alliant Energy [Member]
Notional Amount of Derivatives [Line Items]
Notional Unit Amount Of Derivatives (in MWhs/MWs/Dths/Tons)	0
2015 [Member] | Natural Gas (Dths) [Member] | IPL [Member]
Notional Amount of Derivatives [Line Items]
Notional Unit Amount Of Derivatives (in MWhs/MWs/Dths/Tons)	0
2015 [Member] | Natural Gas (Dths) [Member] | WPL [Member]
Notional Amount of Derivatives [Line Items]
Notional Unit Amount Of Derivatives (in MWhs/MWs/Dths/Tons)	0
2015 [Member] | Coal (Tons) [Member] | Alliant Energy [Member]
Notional Amount of Derivatives [Line Items]
Notional Unit Amount Of Derivatives (in MWhs/MWs/Dths/Tons)	561,000
2015 [Member] | Coal (Tons) [Member] | WPL [Member]
Notional Amount of Derivatives [Line Items]
Notional Unit Amount Of Derivatives (in MWhs/MWs/Dths/Tons)	561,000