Income Taxes (Summary Of Tax Credit Carryforwards) (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2012
Alliant Energy [Member]
Operating Loss Carryforwards [Line Items]
Net operating losses, deferred tax assets	$ 429
IPL [Member]
Operating Loss Carryforwards [Line Items]
Net operating losses, deferred tax assets	155
WPL [Member]
Operating Loss Carryforwards [Line Items]
Net operating losses, deferred tax assets	163
Federal [Member] | Alliant Energy [Member]
Operating Loss Carryforwards [Line Items]
Net operating losses, carryforward amount	828
Net operating losses, deferred tax assets	284
Net operating losses, earliest expiration date	2028
Tax credits, carryforward amount	128
Tax credits, deferred tax asset	126
Tax credits, earliest expiration date	2022
Federal [Member] | IPL [Member]
Operating Loss Carryforwards [Line Items]
Net operating losses, carryforward amount	354
Net operating losses, deferred tax assets	121
Net operating losses, earliest expiration date	2028
Tax credits, carryforward amount	34
Tax credits, deferred tax asset	34
Tax credits, earliest expiration date	2022
Federal [Member] | WPL [Member]
Operating Loss Carryforwards [Line Items]
Net operating losses, carryforward amount	379
Net operating losses, deferred tax assets	130
Net operating losses, earliest expiration date	2028
Tax credits, carryforward amount	36
Tax credits, deferred tax asset	35
Tax credits, earliest expiration date	2022
Federal [Member] | Offset-Uncertain Tax Positions [Member] | Alliant Energy [Member]
Operating Loss Carryforwards [Line Items]
Net operating losses, carryforward amount	(55)
Net operating losses, deferred tax assets	(19)
Federal [Member] | Offset-Uncertain Tax Positions [Member] | IPL [Member]
Operating Loss Carryforwards [Line Items]
Net operating losses, carryforward amount	(25)
Net operating losses, deferred tax assets	(9)
Federal [Member] | Offset-Uncertain Tax Positions [Member] | WPL [Member]
Operating Loss Carryforwards [Line Items]
Net operating losses, carryforward amount	(30)
Net operating losses, deferred tax assets	(10)
State [Member] | Alliant Energy [Member]
Operating Loss Carryforwards [Line Items]
Net operating losses, carryforward amount	776
Net operating losses, deferred tax assets	40
Net operating losses, earliest expiration date	2014
State [Member] | IPL [Member]
Operating Loss Carryforwards [Line Items]
Net operating losses, carryforward amount	170
Net operating losses, deferred tax assets	9
Net operating losses, earliest expiration date	2022
State [Member] | WPL [Member]
Operating Loss Carryforwards [Line Items]
Net operating losses, carryforward amount	183
Net operating losses, deferred tax assets	10
Net operating losses, earliest expiration date	2022
State [Member] | Offset-Uncertain Tax Positions [Member] | Alliant Energy [Member]
Operating Loss Carryforwards [Line Items]
Net operating losses, carryforward amount	(26)
Net operating losses, deferred tax assets	(2)
State [Member] | Offset-Uncertain Tax Positions [Member] | WPL [Member]
Operating Loss Carryforwards [Line Items]
Net operating losses, carryforward amount	(26)
Net operating losses, deferred tax assets	$ (2)