Benefit Plans (Funded Status Of Benefits Plans - Directly Allocated Defined Benefit Pension Plans) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009
Defined Benefit Plan Disclosure [Line Items]
Pension and other benefit obligations	$ (312.7)		$ (303.8)
Regulatory Liabilities	910.1		800.1
Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	126.0
IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	61.0
Pension and other benefit obligations	(101.9)		(110.2)
Regulatory Liabilities	721.3		627.9
WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	52.0
Pension and other benefit obligations	(128.0)		(119.2)
Regulatory Liabilities	188.8		172.2
Directly Assigned Defined Benefit Pension Benefits [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Net projected benefit obligation at Jan. 1	144.4		138.8
Interest cost	7.3		7.8		8.7
Plan amendments	2.8
Actuarial loss	13.2		8.0
Gross benefits paid	(12.3)		(10.2)
Net projected benefit obligation at Dec. 31	155.4		144.4		138.8
Beginning balance, Jan. 1	127.6		121.6
Actual return on plan assets	4.2		15.1
Employer contributions	7.9		1.1
Gross benefits paid	(12.3)		(10.2)
Ending balance, Dec. 31	127.4		127.6		121.6
Under funded status at Dec. 31	(28.0)		(16.8)
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 credit	(3.4)		(3.6)
Amounts recognized in Regulatory Assets and AOCL	72.8		56.9
Accumulated benefit obligations	155.4		144.4
Directly Assigned Defined Benefit Pension Benefits [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Net projected benefit obligation at Jan. 1	107.4		100.9
Interest cost	5.5	[1]	5.6	[1]	6.1	[1]
Plan amendments	0.7
Actuarial loss	10.3		8.7
Gross benefits paid	(8.7)		(7.8)
Net projected benefit obligation at Dec. 31	115.2		107.4		100.9
Beginning balance, Jan. 1	95.6		92.0
Actual return on plan assets	3.2		11.2
Employer contributions	6.9		0.2
Gross benefits paid	(8.7)		(7.8)
Ending balance, Dec. 31	97.0		95.6		92.0
Under funded status at Dec. 31	(18.2)		(11.8)
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 credit	(2.4)		(2.6)
Amounts recognized in Regulatory Assets and AOCL	71.3		59.7
Accumulated benefit obligations	115.2		107.4
Other Pension Plans, Postretirement or Supplemental Plans, Defined Benefit [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Regulatory Liabilities	$ 2.8

[1] In May 2011, Alliant Energy, IPL and WPL amended their defined benefit postretirement health care plans resulting in a revision to the method and level of coverage provided for participants more than 65 years of age. This amendment was determined to be a significant event, which required Alliant Energy, IPL and WPL to remeasure their defined benefit postretirement health care plans in May 2011. The amendment resulted in a decrease in Alliant Energy's, IPL's and WPL's postretirement benefit obligations of $55 million, $30 million and $16 million, respectively, in 2011 with the impact of the remeasurement on net periodic benefit costs being recognized prospectively from the remeasurement date. The impact of the remeasurement decreased Alliant Energy's, IPL's and WPL's net periodic benefit costs by $11.3 million, $7.2 million, and $3.8 million in 2011, respectively. The discount rate used for the remeasurement was 5.20%. All other assumptions used for the remeasurement were consistent with the measurement assumptions used at Dec. 31, 2010.