Income Taxes (Summary Of Tax Credit Carryforwards) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended					12 Months Ended										12 Months Ended
	Dec. 31, 2011	Dec. 31, 2011 Alliant Energy Corporation [Member]		Dec. 31, 2010 Alliant Energy Corporation [Member]		Dec. 31, 2011 Federal [Member] Alliant Energy Corporation [Member]		Dec. 31, 2011 Federal [Member] IPL [Member]	Dec. 31, 2011 Federal [Member] WPL [Member]	Dec. 31, 2011 Federal [Member] General Business Credits [Member] Alliant Energy Corporation [Member]		Dec. 31, 2011 Federal [Member] Alternative Minimum Tax [Member] Alliant Energy Corporation [Member]		Dec. 31, 2011 Federal [Member] Offset Uncertain Tax Positions [Member] Alliant Energy Corporation [Member]		Dec. 31, 2011 State [Member] Alliant Energy Corporation [Member]		Dec. 31, 2011 State [Member] Offset Uncertain Tax Positions [Member] Alliant Energy Corporation [Member]
Operating Loss Carryforwards [Line Items]
Net operating losses, Carryforward Amount						$ 1,034								$ (55)		$ 786	[1]	$ (26)	[1]
Deferred Tax Assets, Federal net operating loss						355	[2]							(19)	[2]	42	[1],[2]	(2)	[1],[2]
Operating loss carryforwards, expiration dates	the state net operating losses carryforwards had expiration dates ranging from 2014 to 2031 with 99% expiring after 2020.					2030		2030	2030							2014	[1]
Federal credits, Carryforward Amount										67		42
Federal credits, Deferred Tax Assets										65	[2]	42	[2]
Federal credits, Earliest Expiration Date										2022		None
Total deferred tax (assets) and liabilities, valuation allowance		$ 1.2	[3]	$ 18.5	[3]

[1] At Dec. 31, 2011, the state net operating losses carryforwards had expiration dates ranging from 2014 to 2031 with 99% expiring after 2020. Due to the uncertainty of the realization of state net operating losses tax carryforwards, Alliant Energy had valuation allowances of $1.2 million and $18.5 million as of Dec. 31, 2011 and 2010, respectively. Refer to "Wisconsin tax legislation" above for discussion of the reversal of valuation allowances related to state net operating loss carryforwards in 2011 as a result of a Wisconsin tax law change.

[2] The amounts for IPL and WPL are included in the deferred tax (assets) and liabilities tables above.

[3] Refer to "Wisconsin tax legislation" above for discussion of the reversal of valuation allowances related to state net operating loss carryforwards in 2011 as a result of a Wisconsin tax law change.