Commitments And Contingencies (Operating Expense Purchase Obligations) (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2012
Long-term Purchase Commitment [Line Items]
Individual commitments incurred	$ 1
Scenario, Forecast [Member] | Alliant Energy [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	1,131
Scenario, Forecast [Member] | IPL [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	605
Scenario, Forecast [Member] | WPL [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	355
Scenario, Forecast [Member] | Other [Member] | Alliant Energy [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	22	[1]
Scenario, Forecast [Member] | Other [Member] | IPL [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	9	[1]
Scenario, Forecast [Member] | Other [Member] | WPL [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	13	[1]
Scenario, Forecast [Member] | Natural Gas [Member] | Alliant Energy [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	344
Scenario, Forecast [Member] | Natural Gas [Member] | IPL [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	208
Scenario, Forecast [Member] | Natural Gas [Member] | WPL [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	136
Scenario, Forecast [Member] | Coal [Member] | Alliant Energy [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	302	[2]
Scenario, Forecast [Member] | Coal [Member] | IPL [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	79	[2]
Scenario, Forecast [Member] | Coal [Member] | WPL [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	52	[2]
Scenario, Forecast [Member] | Coal [Member] | Corporate Services [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	171
Scenario, Forecast [Member] | SO2 emission allowances [Member] | Alliant Energy [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	34
Scenario, Forecast [Member] | SO2 emission allowances [Member] | IPL [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	34
Scenario, Forecast [Member] | SO2 emission allowances [Member] | WPL [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	0
Scenario, Forecast [Member] | Purchased Power [Member] | Alliant Energy [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	429	[3]
Scenario, Forecast [Member] | Purchased Power [Member] | IPL [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	275	[3]
Scenario, Forecast [Member] | Purchased Power [Member] | WPL [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	154	[3]
Scenario, Forecast [Member] | Purchased Power [Member] | Duane Arnold Energy Center (DAEC) (IPL) [Member] | Alliant Energy [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	271	[3]
Scenario, Forecast [Member] | Purchased Power [Member] | Duane Arnold Energy Center (DAEC) (IPL) [Member] | IPL [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	271	[3]
Scenario, Forecast [Member] | Purchased Power [Member] | Duane Arnold Energy Center (DAEC) (IPL) [Member] | WPL [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	0	[3]
Scenario, Forecast [Member] | Purchased Power [Member] | Kewaunee Nuclear Power Plant (Kewaunee) WPL [Member] | Alliant Energy [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	95	[3]
Scenario, Forecast [Member] | Purchased Power [Member] | Kewaunee Nuclear Power Plant (Kewaunee) WPL [Member] | IPL [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	0	[3]
Scenario, Forecast [Member] | Purchased Power [Member] | Kewaunee Nuclear Power Plant (Kewaunee) WPL [Member] | WPL [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	95	[3]
Scenario, Forecast [Member] | Purchased Power [Member] | Other [Member] | Alliant Energy [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	63	[3]
Scenario, Forecast [Member] | Purchased Power [Member] | Other [Member] | IPL [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	4	[3]
Scenario, Forecast [Member] | Purchased Power [Member] | Other [Member] | WPL [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	$ 59	[3]

[1] Includes individual commitments incurred during the normal course of business that exceeded $1 million at September 30, 2012.

[2] Corporate Services entered into system-wide coal contracts on behalf of IPL and WPL that include minimum future commitments of $171 million that have not been directly assigned to IPL and WPL since the specific needs of each utility were not yet known as of September 30, 2012.

[3] Includes payments required by PPAs for capacity rights and minimum quantities of MWhs required to be purchased. Excludes contracts that are considered operating leases.