Income Taxes (Summary Of Tax Credit Carryforwards) (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended
	Jun. 30, 2012
Alliant Energy [Member]
Operating Loss Carryforwards [Line Items]
Net operating losses, Deferred Tax Assets	$ 490
IPL [Member]
Operating Loss Carryforwards [Line Items]
Net operating losses, Deferred Tax Assets	195
WPL [Member]
Operating Loss Carryforwards [Line Items]
Net operating losses, Deferred Tax Assets	177
Federal [Member] | Alliant Energy [Member]
Operating Loss Carryforwards [Line Items]
Net operating losses, Carryforward Amount	1,026
Net operating losses, Deferred Tax Assets	352
Net operating losses, Earliest Expiration Date	2028
Tax credits, Carryforward Amount	122
Tax credits, Deferred Tax Asset	121
Tax credits, Earliest Expiration Date	2022
Federal [Member] | IPL [Member]
Operating Loss Carryforwards [Line Items]
Net operating losses, Carryforward Amount	472
Net operating losses, Deferred Tax Assets	162
Net operating losses, Earliest Expiration Date	2028
Tax credits, Carryforward Amount	32
Tax credits, Deferred Tax Asset	32
Tax credits, Earliest Expiration Date	2022
Federal [Member] | WPL [Member]
Operating Loss Carryforwards [Line Items]
Net operating losses, Carryforward Amount	437
Net operating losses, Deferred Tax Assets	150
Net operating losses, Earliest Expiration Date	2028
Tax credits, Carryforward Amount	33
Tax credits, Deferred Tax Asset	33
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	(57)
Net operating losses, Deferred Tax Assets	(20)
Federal [Member] | Offset-Uncertain Tax Positions [Member] | IPL [Member]
Operating Loss Carryforwards [Line Items]
Net operating losses, Carryforward Amount	(26)
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	(31)
Net operating losses, Deferred Tax Assets	(11)
State [Member] | Alliant Energy [Member]
Operating Loss Carryforwards [Line Items]
Net operating losses, Carryforward Amount	756
Net operating losses, Deferred Tax Assets	39
Net operating losses, Earliest Expiration Date	2014
State [Member] | IPL [Member]
Operating Loss Carryforwards [Line Items]
Net operating losses, Carryforward Amount	171
Net operating losses, Deferred Tax Assets	10
Net operating losses, Earliest Expiration Date	2022
State [Member] | WPL [Member]
Operating Loss Carryforwards [Line Items]
Net operating losses, Carryforward Amount	142
Net operating losses, Deferred Tax Assets	7
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	(27)
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	(27)
Net operating losses, Deferred Tax Assets	$ (2)