Benefit Plans (Funded Status Of Benefits Plans) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009
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
WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	52.0
Defined Benefit Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Net projected benefit obligation at Jan. 1	953.0		920.1
Service cost	11.4		11.9
Interest cost	52.0		52.3
Plan amendments	10.2	[1]
Actuarial (gain) loss	126.2		27.0
Gross benefits paid	(71.4)		(58.3)
Net projected benefit obligation at Dec. 31	1,081.4		953.0
Beginning balance, Jan. 1	823.0		776.4
Actual return on plan assets	28.9		96.9
Employer contributions	116.9		8.0
Gross benefits paid	(71.4)		(58.3)
Ending balance, Dec. 31	897.4		823.0
Under funded status at Dec. 31	(184.0)		(130.0)
Defined Benefit Pension Plans [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	11.4		11.9		11.9
Interest cost	52.0		52.3		54.3
Special termination benefits					0.9	[2]
Beginning balance, Jan. 1	823.0
Ending balance, Dec. 31	897.4		823.0
Defined Benefit Pension Plans [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Net projected benefit obligation at Jan. 1	293.8		284.3
Service cost	6.1		6.2		6.1
Interest cost	16.7		16.5		16.1
Actuarial (gain) loss	45.2		3.1
Gross benefits paid	(17.3)		(16.3)
Net projected benefit obligation at Dec. 31	344.5		293.8		284.3
Beginning balance, Jan. 1	256.9		242.8
Actual return on plan assets	9.1		30.4
Employer contributions	50.0
Gross benefits paid	(17.3)		(16.3)
Ending balance, Dec. 31	298.7		256.9		242.8
Under funded status at Dec. 31	(45.8)		(36.9)
Defined Benefit Pension Plans [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Net projected benefit obligation at Jan. 1	283.3		271.6
Service cost	4.5		4.9		4.9
Interest cost	16.1		15.7		15.5
Actuarial (gain) loss	43.3		2.5
Gross benefits paid	(14.7)		(11.4)
Special termination benefits					0.9	[2]
Net projected benefit obligation at Dec. 31	332.5		283.3		271.6
Beginning balance, Jan. 1	255.2		236.6
Actual return on plan assets	9.1		30.0
Employer contributions	40.0
Gross benefits paid	(14.7)		(11.4)
Ending balance, Dec. 31	289.6		255.2		236.6
Under funded status at Dec. 31	(42.9)		(28.1)
Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Net projected benefit obligation at Jan. 1	274.9		268.4
Service cost	7.0		9.3
Interest cost	12.3		14.9
Plan participants' contributions	6.4		5.6
Plan amendments	(56.6)	[1]	(3.1)	[1]
Actuarial (gain) loss	(0.8)		(0.2)
Early Retiree Reinsurance Program (ERRP) proceeds	0.6
Gross benefits paid	(20.8)		(21.2)
Federal subsidy on other postretirement benefits paid	1.2		1.2
Net projected benefit obligation at Dec. 31	224.2		274.9
Beginning balance, Jan. 1	122.7		102.4
Actual return on plan assets	2.6		14.0
Employer contributions	9.5		21.9
Plan participants' contributions	6.4		5.6
Gross benefits paid	(20.8)		(21.2)
Ending balance, Dec. 31	120.4		122.7
Under funded status at Dec. 31	(103.8)		(152.2)
Other Postretirement Benefits Plans [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	7.0	[3]	9.3	[3]	8.7	[3]
Interest cost	12.3	[3]	14.9	[3]	15.2	[3]
Beginning balance, Jan. 1	122.7
Ending balance, Dec. 31	120.4		122.7
Other Postretirement Benefits Plans [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Net projected benefit obligation at Jan. 1	128.5		123.9
Service cost	2.6	[3]	3.4	[3]	3.1	[3]
Interest cost	5.5	[3]	6.8	[3]	7.3	[3]
Plan participants' contributions	2.0		1.6
Plan amendments	(30.1)	[1]	(1.7)	[1]
Actuarial (gain) loss	(2.7)		4.0
Early Retiree Reinsurance Program (ERRP) proceeds	0.2
Gross benefits paid	(9.1)		(10.1)
Federal subsidy on other postretirement benefits paid	0.6		0.6
Net projected benefit obligation at Dec. 31	97.5		128.5		123.9
Beginning balance, Jan. 1	77.5		68.4
Actual return on plan assets	1.4		9.6
Employer contributions	2.9		8.0
Plan participants' contributions	2.0		1.6
Gross benefits paid	(9.1)		(10.1)
Ending balance, Dec. 31	74.7		77.5		68.4
Under funded status at Dec. 31	(22.8)		(51.0)
Other Postretirement Benefits Plans [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Net projected benefit obligation at Jan. 1	103.3		99.1
Service cost	2.9	[3]	3.6	[3]	3.4	[3]
Interest cost	4.9	[3]	5.5	[3]	5.5	[3]
Plan participants' contributions	3.3		3.2
Plan amendments	(18.2)	[1]	(1.5)	[1]
Actuarial (gain) loss	2.3		1.9
Early Retiree Reinsurance Program (ERRP) proceeds	0.2
Gross benefits paid	(9.6)		(9.0)
Federal subsidy on other postretirement benefits paid	0.5		0.5
Net projected benefit obligation at Dec. 31	89.6		103.3		99.1
Beginning balance, Jan. 1	25.0		18.8
Actual return on plan assets	0.9		2.1
Employer contributions	5.5		9.9
Plan participants' contributions	3.3		3.2
Gross benefits paid	(9.6)		(9.0)
Ending balance, Dec. 31	25.1		25.0		18.8
Under funded status at Dec. 31	$ (64.5)		$ (78.3)

[1] Refer to "Net Periodic Benefit Costs" above for additional information regarding plan amendments to the defined benefit pension and other postretirement benefits plans in 2011.

[2] In 2009, Alliant Energy eliminated certain corporate and operations positions. As a result, Alliant Energy recognized curtailment losses related to its pension plans of $0.3 million in 2009. In addition, Alliant Energy and WPL recognized special termination benefits costs related to the qualified defined benefit pension plan that is sponsored by WPL of $0.9 million in 2009.

[3] 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.