Benefit Plans (Defined Benefit Pension And Other Postretirement Benefits Plans) (Details) (USD $) In Millions	3 Months Ended				6 Months Ended	9 Months Ended
	Sep. 30, 2011		Sep. 30, 2010		Dec. 31, 2011	Sep. 30, 2011		Sep. 30, 2010
IPL [Member] | Other Postretirement Benefit Plans [Member]
Service cost	$ 0.6	[1]	$ 0.8	[1]		$ 2.0	[1]	$ 2.6	[1]
Interest cost	1.3	[1]	1.6	[1]		4.3	[1]	5.2	[1]
Expected return on plan assets	(1.4)	[1]	(1.3)	[1]		(4.0)	[1]	(4.0)	[1]
Transition obligation								0.1	[1]
Prior service cost (credit)	(1.8)	[1]	(0.5)	[1]		(3.3)	[1]	(0.6)	[1]
Actuarial loss	0.7	[1]	1.2	[1]		2.2	[1]	2.8	[1]
Total	(0.6)	[1]	1.8	[1]		1.2	[1]	6.1	[1]
IPL [Member] | Defined Benefit Pension Plans [Member]
Service cost	1.5		1.5			4.6		4.6
Interest cost	4.2		4.2			12.5		12.4
Expected return on plan assets	(5.0)		(4.9)			(15.0)		(14.6)
Prior service cost (credit)	0.2		0.2			0.4		0.5
Actuarial loss	1.4		1.8			4.3		5.4
Additional benefit costs	0.8					6.3
Total	2.3		2.8			6.8		8.3
WPL [Member] | Other Postretirement Benefit Plans [Member]
Service cost	0.7	[1]	0.9	[1]		2.2	[1]	2.7	[1]
Interest cost	1.1	[1]	1.3	[1]		3.7	[1]	4.2	[1]
Expected return on plan assets	(0.3)	[1]	(0.4)	[1]		(1.0)	[1]	(1.0)	[1]
Prior service cost (credit)	(1.1)	[1]	(0.2)	[1]		(2.2)	[1]	(0.2)	[1]
Actuarial loss	0.5	[1]	0.7	[1]		1.6	[1]	1.7	[1]
Total	0.9	[1]	2.3	[1]		4.3	[1]	7.4	[1]
WPL [Member] | Defined Benefit Pension Plans [Member]
Service cost	1.1		1.2			3.4		3.6
Interest cost	4.1		3.9			12.1		11.8
Expected return on plan assets	(5.0)		(4.8)			(15.0)		(14.3)
Prior service cost (credit)	0.1		0.2			0.4		0.4
Actuarial loss	1.8		2.2			5.3		6.4
Additional benefit costs	0.4					3.4
Total	2.1		2.7			6.2		7.9
Defined Benefit Pension Plans [Member]
Service cost	2.9		3.0			8.6		9.0
Interest cost	13.0		13.1			39.0		39.3
Expected return on plan assets	(15.9)		(15.6)			(47.8)		(46.6)
Prior service cost (credit)	0.2		0.2			0.6		0.6
Actuarial loss	5.2		5.9			15.7		17.9
Additional benefit costs	1.3	[2]			10.2	10.2	[2]
Settlement losses	1.1	[3]	1.4	[3]		1.1	[3]	1.4	[3]
Total	7.8		8.0			27.4		21.6
Other Postretirement Benefit Plans [Member]
Service cost	1.6	[1]	2.2	[1]		5.4	[1]	7.0	[1]
Interest cost	2.8	[1]	3.5	[1]		9.5	[1]	11.4	[1]
Expected return on plan assets	(2.0)	[1]	(1.9)	[1]		(5.9)	[1]	(5.8)	[1]
Transition obligation								0.1	[1]
Prior service cost (credit)	(3.4)	[1]	(1.2)	[1]		(6.6)	[1]	(1.3)	[1]
Actuarial loss	1.2	[1]	2.2	[1]		4.0	[1]	5.3	[1]
Total	$ 0.2	[1]	$ 4.8	[1]		$ 6.4	[1]	$ 16.7	[1]

[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 remeasurement resulted in a decrease in Alliant Energy's postretirement benefit obligations of $64 million ($38 million for IPL, $16 million for WPL and $10 million for Corporate Services) 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 $4.3 million, $2.7 million and $1.4 million during the three months ended Sep. 30, 2011, respectively, and $7.1 million, $4.5 million, and $2.4 million during the nine months ended Sep. 30, 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.

[2] Alliant Energy reached an agreement with the IRS, which resulted in a favorable determination letter for the Alliant Energy Cash Balance Pension Plan (Plan) during the first quarter of 2011. The agreement with the IRS required Alliant Energy to amend the Plan, which was completed in the second quarter of 2011 resulting in aggregate additional benefits of $10.2 million to be paid to certain former participants in the Plan in the second half of 2011. Alliant Energy recognized $1.3 million and $10.2 million of additional benefits costs during the three and nine months ended Sep. 30, 2011, respectively, related to these benefits. Refer to Note 11(c) for additional information regarding the Plan.

[3] For both the three and nine months ended Sep. 30, 2011 and 2010, settlement losses of $1.1 million and $1.4 million, respectively, related to payments made to retired executives.