Income Taxes (Summary Of Tax Credit Carryforwards) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012
Alliant Energy Corporation [Member]
Operating Loss Carryforwards [Line Items]
Net operating losses, Deferred Tax Assets	$ 491
IPL [Member]
Operating Loss Carryforwards [Line Items]
Net operating losses, Deferred Tax Assets	194
WPL [Member]
Operating Loss Carryforwards [Line Items]
Net operating losses, Deferred Tax Assets	174
Federal [Member] | Alliant Energy Corporation [Member]
Operating Loss Carryforwards [Line Items]
Net operating losses, Carryforward Amount	1,043
Net operating losses, Deferred Tax Assets	358
Net operating losses, Earliest Expiration Date	2028
Tax credits, Carryforward Amount	116
Tax credits, Deferred Tax Asset	114
Tax credits, Earliest Expiration Date	2022
Federal [Member] | IPL [Member]
Operating Loss Carryforwards [Line Items]
Net operating losses, Carryforward Amount	475
Net operating losses, Deferred Tax Assets	163
Net operating losses, Earliest Expiration Date	2028
Tax credits, Carryforward Amount	29
Tax credits, Deferred Tax Asset	29
Tax credits, Earliest Expiration Date	2022
Federal [Member] | WPL [Member]
Operating Loss Carryforwards [Line Items]
Net operating losses, Carryforward Amount	441
Net operating losses, Deferred Tax Assets	151
Net operating losses, Earliest Expiration Date	2028
Tax credits, Carryforward Amount	30
Tax credits, Deferred Tax Asset	29
Tax credits, Earliest Expiration Date	2022
Federal [Member] | Offset-Uncertain Tax Positions [Member] | Alliant Energy Corporation [Member]
Operating Loss Carryforwards [Line Items]
Net operating losses, Carryforward Amount	(56)
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	(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	(31)
Net operating losses, Deferred Tax Assets	(11)
State [Member] | Alliant Energy Corporation [Member]
Operating Loss Carryforwards [Line Items]
Net operating losses, Carryforward Amount	783
Net operating losses, Deferred Tax Assets	41
Net operating losses, Earliest Expiration Date	2014
State [Member] | IPL [Member]
Operating Loss Carryforwards [Line Items]
Net operating losses, Carryforward Amount	189
Net operating losses, Deferred Tax Assets	11
Net operating losses, Earliest Expiration Date	2022
State [Member] | WPL [Member]
Operating Loss Carryforwards [Line Items]
Net operating losses, Carryforward Amount	148
Net operating losses, Deferred Tax Assets	7
Net operating losses, Earliest Expiration Date	2022
State [Member] | Offset-Uncertain Tax Positions [Member] | Alliant Energy Corporation [Member]
Operating Loss Carryforwards [Line Items]
Net operating losses, Carryforward Amount	(28)
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	(28)
Net operating losses, Deferred Tax Assets	$ (2)