Income Taxes (Schedule Of Uncertain Tax Positions Balances In Regulatory Liabilities) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009		Dec. 31, 2008
Alliant Energy Corporation [Member]
Tax positions favorably impacting future effective tax rates for continuing operations					$ 13.6
Interest accrued	0.4		0.7		5.0
Penalties accrued
Uncertain tax positions balances	23.5	[1]	66.7	[1]	101.7	[1]	14.0
IPL [Member]
Tax positions favorably impacting future effective tax rates for continuing operations					10.9
Interest accrued	0.4		0.7		2.8
Penalties accrued
Uncertain tax positions balances	10.9	[1]	33.0	[1]	58.4	[1]	9.4
WPL [Member]
Tax positions favorably impacting future effective tax rates for continuing operations					1.4
Interest accrued					2.1
Penalties accrued
Uncertain tax positions balances	12.6	[1]	33.7	[1]	42.1	[1]	2.5
Regulatory Liabilities [Member] | Alliant Energy And IPL [Member]
Uncertain tax positions balances	$ 10.0		$ 31.0

[1] At Dec. 31, 2011 and 2010, $10 million and $31 million, respectively, of uncertain tax positions balances for both Alliant Energy and IPL included amounts recorded in regulatory liability accounts.