Benefit Plans (Amounts Recognized On The Consolidated Balance Sheets) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011		Dec. 31, 2010
Defined Benefit Plan Disclosure [Line Items]
Pension and other benefit obligations	$ (312.7)		$ (303.8)
IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and other benefit obligations	(101.9)		(110.2)
WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and other benefit obligations	(128.0)		(119.2)
Defined Benefit Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss	494.8	[1]	356.0	[1]
Prior service cost (credit)	(6.9)	[1]	(6.2)	[1]
Amounts recognized in Regulatory Assets, Regulatory Liabilities and AOCL	487.9	[1]	349.8	[1]
Defined Benefit Pension Plans [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and other benefit obligations	(45.8)		(36.9)
Net actuarial loss	141.1	[2]	90.8	[2]
Prior service cost (credit)	1.1	[2]	1.6	[2]
Amounts recognized in Regulatory Assets and Regulatory Liabilities	142.2	[2]	92.4	[2]
Defined Benefit Pension Plans [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and other benefit obligations	(42.9)		(28.1)
Net amount recognized at Dec. 31	(42.9)		(28.1)
Amounts recognized in Regulatory Assets, Regulatory Liabilities and AOCL	145.1		98.5
Net actuarial loss	142.7		95.6
Prior service cost (credit)	2.4		2.9
Directly Assigned Defined Benefit Pension Benefits [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Other current liabilities	(0.8)		(1.0)
Pension and other benefit obligations	(27.2)		(15.8)
Net amount recognized at Dec. 31	28.0		16.8
Net actuarial loss	76.2		60.5
Prior service cost (credit)	(3.4)		(3.6)
Directly Assigned Defined Benefit Pension Benefits [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Other current liabilities	(0.2)		(0.1)
Pension and other benefit obligations	(18.0)		(11.7)
Net amount recognized at Dec. 31	18.2		11.8
Net actuarial loss	73.7		62.3
Prior service cost (credit)	(2.4)		(2.6)
Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss	76.7	[1]	77.1	[1]
Prior service cost (credit)	(52.5)	[1]	(5.7)	[1]
Amounts recognized in Regulatory Assets, Regulatory Liabilities and AOCL	24.2	[1]	71.4	[1]
Defined benefit plan regulatory liability	3.3
Other Postretirement Benefits Plans [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and other benefit obligations	(22.8)		(51.0)
Net actuarial loss	41.0	[2]	42.4	[2]
Prior service cost (credit)	(27.6)	[2]	(2.5)	[2]
Amounts recognized in Regulatory Assets and Regulatory Liabilities	13.4	[2]	39.9	[2]
Other Postretirement Benefits Plans [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Non-current assets	1.3
Pension and other benefit obligations	(65.8)		(78.3)
Net amount recognized at Dec. 31	(64.5)		(78.3)
Amounts recognized in Regulatory Assets, Regulatory Liabilities and AOCL	10.5		24.7
Net actuarial loss	27.8		27.2
Prior service cost (credit)	$ (17.3)		$ (2.5)

[1] Refer to Note 1(b) and Alliant Energy's Consolidated Statements of Common Equity for amounts recognized in "Regulatory assets" and "AOCL," respectively, on Alliant Energy's Consolidated Balance Sheets. At Dec. 31, 2011, $3.3 million of regulatory liabilities were recognized related to Alliant Energy's other postretirement benefits plans.

[2] At Dec. 31, 2011, $2.8 million of regulatory liabilities were recognized related to IPL's other postretirement benefits plans.