Benefit Plans (Defined Benefit Pension And Other Postretirement Benefits Plans) (Details) (USD $) In Millions, unless otherwise specified	1 Months Ended	12 Months Ended
	May 31, 2011	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009
Defined Benefit Plan Disclosure [Line Items]
Increase pension and other postretirement benefits obligations		$ 8.9		$ (20.1)		$ (190.0)
Discount rate used for remeasurement		5.20%
Defined Benefit Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost		11.4		11.9
Interest cost		52.0		52.3
Additional benefit costs		10.2
Curtailment losses						0.3
Discount rate used for remeasurement		4.86%		5.56%		5.80%
Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost		7.0		9.3
Interest cost		12.3		14.9
Increase pension and other postretirement benefits obligations	55.0
Decrease in net periodic benefit costs		11.3
Discount rate used for remeasurement		4.60%		5.25%		5.55%
Cash Balance Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Additional benefit costs		10.2
Alliant Energy Corporation [Member] | Defined Benefit Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost		11.4		11.9		11.9
Interest cost		52.0		52.3		54.3
Expected return on plan assets		(63.8)	[1]	(62.1)	[1]	(47.6)	[1]
Prior service cost (credit)		0.7	[2]	0.9	[2]	2.1	[2]
Actuarial loss		21.1		23.8		30.6
Additional benefit costs		10.2	[3]
Settlement losses		1.1	[4]	1.4	[4]
Curtailment losses						1.0	[5],[6]
Special termination benefits costs						0.9	[6]
Total		32.7		28.2		53.2
Alliant Energy Corporation [Member] | Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost		7.0	[7]	9.3	[7]	8.7	[7]
Interest cost		12.3	[7]	14.9	[7]	15.2	[7]
Expected return on plan assets		(7.9)	[1],[7]	(7.7)	[1],[7]	(6.1)	[1],[7]
Transition obligation				0.1	[2],[7]	0.2	[2],[7]
Prior service cost (credit)		(10.0)	[2],[7]	(2.4)	[2],[7]	(3.7)	[2],[7]
Actuarial loss		5.3	[7]	7.4	[7]	6.2	[7]
Total		6.7	[7]	21.6	[7]	20.5	[7]
IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Increase pension and other postretirement benefits obligations		(8.3)		(9.0)		(94.7)
IPL [Member] | Corporate Service Allocation [Member]
Defined Benefit Plan Disclosure [Line Items]
Additional benefit costs		3.5
IPL [Member] | Defined Benefit Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost		6.1		6.2		6.1
Interest cost		16.7		16.5		16.1
Expected return on plan assets		(20.0)	[1]	(19.5)	[1]	(14.0)	[1]
Prior service cost (credit)		0.5	[2]	0.6	[2]	0.8	[2]
Actuarial loss		5.7		7.2		9.3
Additional benefit costs		6.3
Total		9.0		11.0		18.3
Discount rate used for remeasurement		4.95%		5.70%		5.80%
IPL [Member] | Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost		2.6	[7]	3.4	[7]	3.1	[7]
Interest cost		5.5	[7]	6.8	[7]	7.3	[7]
Expected return on plan assets		(5.4)	[1],[7]	(5.3)	[1],[7]	(4.2)	[1],[7]
Transition obligation				0.1	[2],[7]	0.2	[2],[7]
Prior service cost (credit)		(5.0)	[2],[7]	(1.1)	[2],[7]	(1.6)	[2],[7]
Actuarial loss		2.9	[7]	4.0	[7]	4.0	[7]
Total		0.6	[7]	7.9	[7]	8.8	[7]
Increase pension and other postretirement benefits obligations	30.0
Decrease in net periodic benefit costs		7.2
Discount rate used for remeasurement		4.60%		5.25%		5.55%
IPL [Member] | Directly Assigned Defined Benefit Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost		7.3		7.8		8.7
Expected return on plan assets		(9.7)	[1]	(9.7)	[1]	(8.4)	[1]
Prior service cost (credit)		(0.2)	[2]	(0.3)	[2]	(0.1)	[2]
Actuarial loss		3.0	[2]	2.9	[2]	4.2	[2]
Additional benefit costs		2.8	[3]
Curtailment losses						(0.1)
Total		3.2		0.7		4.3
WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Increase pension and other postretirement benefits obligations		8.8		(2.5)		(63.4)
WPL [Member] | Corporate Service Allocation [Member]
Defined Benefit Plan Disclosure [Line Items]
Additional benefit costs		2.7
WPL [Member] | Defined Benefit Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost		4.5		4.9		4.9
Interest cost		16.1		15.7		15.5
Expected return on plan assets		(20.0)	[1]	(19.1)	[1]	(14.1)	[1]
Prior service cost (credit)		0.5	[2]	0.5	[2]	0.5	[2]
Actuarial loss		7.1	[2]	8.5	[2]	10.4	[2]
Additional benefit costs		3.4
Settlement losses						0.9
Curtailment losses						0.7	[5]
Special termination benefits costs						0.9	[6]
Total		8.2		10.5		18.8
Discount rate used for remeasurement		4.95%		5.70%		5.80%
WPL [Member] | Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost		2.9	[7]	3.6	[7]	3.4	[7]
Interest cost		4.9	[7]	5.5	[7]	5.5	[7]
Expected return on plan assets		(1.3)	[1],[7]	(1.3)	[1],[7]	(1.1)	[1],[7]
Prior service cost (credit)		(3.4)	[2],[7]	(0.7)	[2],[7]	(0.9)	[2],[7]
Actuarial loss		2.1	[2],[7]	2.5	[2],[7]	1.3	[2],[7]
Total		5.2	[7]	9.6	[7]	8.2	[7]
Increase pension and other postretirement benefits obligations	16.0
Decrease in net periodic benefit costs		3.8
Discount rate used for remeasurement		4.60%		5.25%		5.55%
WPL [Member] | Directly Assigned Defined Benefit Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost		5.5	[7]	5.6	[7]	6.1	[7]
Expected return on plan assets		(7.3)	[1],[7]	(7.3)	[1],[7]	(6.3)	[1],[7]
Prior service cost (credit)		(0.2)	[2],[7]	(0.2)	[2],[7]
Actuarial loss		3.0	[2],[7]	2.8	[2],[7]	3.6	[2],[7]
Additional benefit costs		0.7	[3],[7]
Total		$ 1.7	[7]	$ 0.9	[7]	$ 3.4	[7]

[1] The expected return on plan assets is based on the expected rate of return on plan assets and the fair value approach to the market-related value of plan assets.

[2] Unrecognized net actuarial gains or losses in excess of 10% of the greater of the plans' benefit obligation or asset are amortized over the average future service lives of plan participants, except for the Alliant Energy Cash Balance Pension Plan (Cash Balance Plan) where gains or losses outside the 10% threshold are amortized over the time period the participants are expected to receive benefits. Unrecognized prior service costs (credits) for the postretirement benefits plans are amortized over the average future service period to full eligibility of the participants of each plan.

[3] Alliant Energy reached an agreement with the IRS, which resulted in a favorable determination letter for the Cash Balance Plan in 2011. The agreement with the IRS required Alliant Energy to amend the Cash Balance Plan, which was completed in 2011 resulting in aggregate additional benefits of $10.2 million paid to certain former participants in the Cash Balance Plan in 2011. Alliant Energy recognized $10.2 million ($6.3 million at IPL ($2.8 million directly assigned and $3.5 million allocated by Corporate Services) and $3.4 million at WPL ($0.7 million directly assigned and $2.7 million allocated by Corporate Services)) of additional benefits costs in 2011 related to these benefits. Refer to Note 13(b) for additional information regarding the Cash Balance Plan.

[4] In 2011 and 2010, the settlement losses of $1.1 million and $1.4 million, respectively, related to payments made to retired executives of Alliant Energy.

[5] In 2007, members of the International Brotherhood of Electrical Workers Local 965 ratified a four-year collective bargaining agreement reached with WPL, resulting in changes to WPL's qualified pension plan. One of these changes provided WPL qualified pension plan participants an option to cease participating in the WPL qualified pension plan and begin participating in the Alliant Energy 401(k) Savings Plan with increased levels of contribution by Alliant Energy. The election of this option did not impact a participant's eligibility for benefits previously vested under the WPL qualified pension plan. In 2009, certain of these employees elected to cease participating in the WPL qualified pension plan, resulting in Alliant Energy and WPL recognizing a curtailment loss related to the WPL qualified pension plan of $0.7 million in 2009.

[6] 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.

[7] 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.