Income Taxes (Net Operating Loss Carryforwards And Associated Deferred Tax Assets) (Details) (USD $) In Millions	Sep. 30, 2011
Federal [Member]
Net operating losses, carryforward amount	$ 860
Net operating losses, deferred tax assets	295	[1]
State [Member]
Net operating losses, carryforward amount	623
Net operating losses, deferred tax assets	32	[1]
IPL [Member] | Federal And State [Member]
Net operating losses, deferred tax assets	108
WPL [Member] | Federal And State [Member]
Net operating losses, deferred tax assets	116
Federal And State [Member]
Net operating losses, deferred tax assets	$ 272

[1] The gross net operating losses are offset by uncertain tax positions that if lost, will be settled in a reduction of net operating losses and not with tax payments. Alliant Energy's, IPL's and WPL's deferred tax assets related to federal and state net operating losses, net of offsets, were $272 million, $108 million and $116 million, respectively.