Benefit Plans (Defined Benefit Pension And Other Postretirement Benefits Plans) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended	3 Months Ended	9 Months Ended	3 Months Ended				9 Months Ended				3 Months Ended				9 Months Ended				3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended				9 Months Ended				3 Months Ended	9 Months Ended	3 Months Ended	9 Months Ended	3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended				9 Months Ended				3 Months Ended	9 Months Ended	3 Months Ended	9 Months Ended
	Dec. 31, 2011 Defined Benefit Pension Plans [Member]	Sep. 30, 2011 Cash Balance Plan [Member]	Sep. 30, 2011 Cash Balance Plan [Member]	Sep. 30, 2012 Alliant Energy [Member] Defined Benefit Pension Plans [Member]		Sep. 30, 2011 Alliant Energy [Member] Defined Benefit Pension Plans [Member]		Sep. 30, 2012 Alliant Energy [Member] Defined Benefit Pension Plans [Member]		Sep. 30, 2011 Alliant Energy [Member] Defined Benefit Pension Plans [Member]		Sep. 30, 2012 Alliant Energy [Member] Other Postretirement Benefits Plans [Member]		Sep. 30, 2011 Alliant Energy [Member] Other Postretirement Benefits Plans [Member]		Sep. 30, 2012 Alliant Energy [Member] Other Postretirement Benefits Plans [Member]		Sep. 30, 2011 Alliant Energy [Member] Other Postretirement Benefits Plans [Member]		Sep. 30, 2012 IPL [Member] Other Postretirement Benefits Plans [Member]	Sep. 30, 2011 IPL [Member] Other Postretirement Benefits Plans [Member]	Sep. 30, 2012 IPL [Member] Other Postretirement Benefits Plans [Member]	Sep. 30, 2011 IPL [Member] Other Postretirement Benefits Plans [Member]	Sep. 30, 2012 IPL [Member] Qualified Defined Benefit Pension Plans [Member]	Sep. 30, 2011 IPL [Member] Qualified Defined Benefit Pension Plans [Member]	Sep. 30, 2012 IPL [Member] Qualified Defined Benefit Pension Plans [Member]	Sep. 30, 2011 IPL [Member] Qualified Defined Benefit Pension Plans [Member]	Sep. 30, 2012 IPL [Member] Directly assigned defined benefit pension plans [Member]		Sep. 30, 2011 IPL [Member] Directly assigned defined benefit pension plans [Member]		Sep. 30, 2012 IPL [Member] Directly assigned defined benefit pension plans [Member]		Sep. 30, 2011 IPL [Member] Directly assigned defined benefit pension plans [Member]		Sep. 30, 2011 IPL [Member] Corporate Service Allocation [Member]	Sep. 30, 2011 IPL [Member] Corporate Service Allocation [Member]	Sep. 30, 2011 IPL [Member] Cash Balance Plan [Member]	Sep. 30, 2011 IPL [Member] Cash Balance Plan [Member]	Sep. 30, 2012 WPL [Member] Other Postretirement Benefits Plans [Member]	Sep. 30, 2011 WPL [Member] Other Postretirement Benefits Plans [Member]	Sep. 30, 2012 WPL [Member] Other Postretirement Benefits Plans [Member]	Sep. 30, 2011 WPL [Member] Other Postretirement Benefits Plans [Member]	Sep. 30, 2012 WPL [Member] Qualified Defined Benefit Pension Plans [Member]	Sep. 30, 2011 WPL [Member] Qualified Defined Benefit Pension Plans [Member]	Sep. 30, 2012 WPL [Member] Qualified Defined Benefit Pension Plans [Member]	Sep. 30, 2011 WPL [Member] Qualified Defined Benefit Pension Plans [Member]	Sep. 30, 2012 WPL [Member] Directly assigned defined benefit pension plans [Member]		Sep. 30, 2011 WPL [Member] Directly assigned defined benefit pension plans [Member]		Sep. 30, 2012 WPL [Member] Directly assigned defined benefit pension plans [Member]		Sep. 30, 2011 WPL [Member] Directly assigned defined benefit pension plans [Member]		Sep. 30, 2011 WPL [Member] Corporate Service Allocation [Member]	Sep. 30, 2011 WPL [Member] Corporate Service Allocation [Member]	Sep. 30, 2011 WPL [Member] Cash Balance Plan [Member]	Sep. 30, 2011 WPL [Member] Cash Balance Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost				$ 3.3		$ 2.9		$ 10.0		$ 8.6		$ 1.7		$ 1.6		$ 5.2		$ 5.4		$ 0.7	$ 0.6	$ 2.2	$ 2.0	$ 1.8	$ 1.5	$ 5.6	$ 4.6													$ 0.7	$ 0.7	$ 2.1	$ 2.2	$ 1.3	$ 1.1	$ 3.9	$ 3.4
Interest cost				13.0		13.0		38.9		39.0		2.6		2.8		7.7		9.5		1.1	1.3	3.3	4.3	4.3	4.2	12.8	12.5	1.8		1.9		5.3		5.6						1.0	1.1	3.1	3.7	4.1	4.1	12.3	12.1	1.3		1.4		3.9		4.1
Expected return on plan assets				(17.2)		(15.9)		(51.6)		(47.8)		(1.9)		(2.0)		(5.7)		(5.9)		(1.3)	(1.4)	(3.9)	(4.0)	(5.8)	(5.0)	(17.3)	(15.0)	(2.4)		(2.4)		(7.2)		(7.3)						(0.3)	(0.3)	(1.0)	(1.0)	(5.6)	(5.0)	(16.8)	(15.0)	(1.9)		(1.9)		(5.5)		(5.5)
Amortization of:
Prior service cost (credit)				0.1		0.2		0.2		0.6		(3.0)		(3.4)		(9.0)		(6.6)		(1.5)	(1.8)	(4.7)	(3.3)	0.1	0.2	0.3	0.4	(0.1)		(0.1)		(0.2)		(0.2)						(1.0)	(1.1)	(2.9)	(2.2)	0.2	0.1	0.4	0.4	0		0		(0.1)		(0.1)
Actuarial loss				8.3		5.2		24.9		15.7		1.6		1.2		4.7		4.0		0.9	0.7	2.7	2.2	2.6	1.4	7.7	4.3	1.0		0.7		2.9		2.2						0.6	0.5	1.7	1.6	3.0	1.8	9.1	5.3	0.9		0.8		2.7		2.2
Additional benefits costs	10.0	1.3	10.2	0	[1]	1.3	[1]	0	[1]	10.2	[1]	0	[1]	0	[1]	0	[1]	0	[1]									0	[1]	0.6	[1]	0	[1]	2.8	[1]	0.2	3.5	0.8	6.3									0	[1]	0.1	[1]	0	[1]	0.7	[1]	0.3	2.7	0.4	3.4
Settlement loss				0		1.1		0		1.1		0		0		0		0
Total				$ 7.5		$ 7.8		$ 22.4		$ 27.4		$ 1.0		$ 0.2		$ 2.9		$ 6.4		$ (0.1)	$ (0.6)	$ (0.4)	$ 1.2	$ 3.0	$ 2.3	$ 9.1	$ 6.8	$ 0.3		$ 0.7		$ 0.8		$ 3.1						$ 1.0	$ 0.9	$ 3.0	$ 4.3	$ 3.0	$ 2.1	$ 8.9	$ 6.2	$ 0.3		$ 0.4		$ 1.0		$ 1.4

[1] Alliant Energy reached an agreement with the IRS, which resulted in a favorable determination letter for the Cash Balance Plan during the first quarter of 2011. The agreement with the IRS required Alliant Energy to amend the Cash Balance Plan, which was completed in the second quarter of 2011, resulting in aggregate additional benefits of $10.2 million paid by Alliant Energy to certain former participants in the Cash Balance 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 September 30, 2011, respectively, related to these benefits. IPL recognized $0.8 million ($0.6 million directly assigned and $0.2 million allocated by Corporate Services) and $6.3 million ($2.8 million directly assigned and $3.5 million allocated by Corporate Services) of additional benefits costs during the three and nine months ended September 30, 2011, respectively, related to these benefits. WPL recognized $0.4 million ($0.1 million directly assigned and $0.3 million allocated by Corporate Services) and $3.4 million ($0.7 million directly assigned and $2.7 million allocated by Corporate Services) of additional benefits costs during the three and nine months ended September 30, 2011, respectively, related to these benefits. Refer to Note 11(c) for additional information regarding the Cash Balance Plan.