Income Taxes (Schedule Of Effective Income Tax Rate Reconciliation) (Details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Alliant Energy Corporation [Member]
Statutory federal income tax rate	35.00%	35.00%	35.00%
State income taxes, net of federal benefits	4.30%	4.70%	7.80%
Adjustment of prior period taxes	0.50%	0.20%	(6.20%)
IPL's tax benefit rider	(9.60%)
Production tax credits	(7.20%)	(2.50%)	(3.90%)
Wisconsin Tax Legislation	(5.00%)
Federal Health Care Legislation		1.60%
State filing changes			(33.80%)
Effect of rate making on property related differences, Federal	(0.10%)	(1.10%)	(2.40%)
Effect of rate making on property related differences, State	$ (2.1)	$ (3.2)	$ (2.4)
Other items, net	(1.10%)	(2.70%)	(1.90%)
Effective income tax rate, continuing operations, total	14.70%	32.00%	(7.80%)
IPL [Member]
Statutory federal income tax rate	35.00%	35.00%	35.00%
State income taxes, net of federal benefits	4.30%	4.10%	5.60%
Adjustment of prior period taxes	1.70%	(1.80%)	(5.40%)
IPL's tax benefit rider	(26.50%)
Production tax credits	(9.10%)	(4.10%)	(0.50%)
Federal Health Care Legislation		2.00%
State filing changes			(18.20%)
Effect of rate making on property related differences, Federal	0.50%	(1.30%)	(1.00%)
Effect of rate making on property related differences, State	(5.8)	(7.6)	(1.6)
Other items, net	(2.80%)	(3.50%)	1.10%
Effective income tax rate, continuing operations, total	(2.70%)	22.80%	15.00%
WPL [Member]
Statutory federal income tax rate	35.00%	35.00%	35.00%
State income taxes, net of federal benefits	5.00%	5.10%	4.50%
Adjustment of prior period taxes		2.00%	0.70%
Production tax credits	(6.00%)	(1.40%)	(2.90%)
Federal Health Care Legislation		1.20%
State filing changes			(1.80%)
Effect of rate making on property related differences, Federal	(0.40%)	(1.00%)	(0.80%)
Effect of rate making on property related differences, State	$ (0.1)	$ (0.1)	$ (0.1)
Other items, net	(0.10%)	(1.60%)	(0.70%)
Effective income tax rate, continuing operations, total	33.40%	39.20%	33.90%