Benefit Plans (Defined Benefit Pension And Other Postretirement Benefits Plans) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2012		Jun. 30, 2011		Jun. 30, 2012		Jun. 30, 2011
Defined Benefit Pension Plans [Member]
Amortization of:
Additional benefit costs							$ 10.0
Cash Balance Plan [Member]
Amortization of:
Additional benefit costs			8.9
Alliant Energy [Member] | Defined Benefit Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	3.4		2.8		6.7		5.7
Interest cost	12.9		13.0		25.9		26.0
Expected return on plan assets	(17.2)		(15.9)		(34.4)		(31.9)
Amortization of:
Prior service cost (credit)	0		0.2		0.1		0.4
Actuarial loss	8.3		5.3		16.6		10.5
Additional benefit costs	0	[1]	8.9	[1]	0	[1]	8.9	[1]
Total	7.4		14.3		14.9		19.6
Alliant Energy [Member] | Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	1.8		1.7		3.5		3.8
Interest cost	2.5		3.1		5.1		6.7
Expected return on plan assets	(1.9)		(2.0)		(3.8)		(3.9)
Amortization of:
Prior service cost (credit)	(3.0)		(2.5)		(6.0)		(3.2)
Actuarial loss	1.5		1.4		3.1		2.8
Additional benefit costs	0	[1]	0	[1]	0	[1]	0	[1]
Total	0.9		1.7		1.9		6.2
IPL [Member] | Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.7		0.6		1.5		1.4
Interest cost	1.1		1.3		2.2		3.0
Expected return on plan assets	(1.3)		(1.3)		(2.6)		(2.6)
Amortization of:
Prior service cost (credit)	(1.6)		(1.2)		(3.2)		(1.5)
Actuarial loss	0.9		0.7		1.8		1.5
Total	(0.2)		0.1		(0.3)		1.8
IPL [Member] | Qualified Defined Benefit Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	1.9		1.5		3.8		3.1
Interest cost	4.2		4.1		8.5		8.3
Expected return on plan assets	(5.7)		(5.0)		(11.5)		(10.0)
Amortization of:
Prior service cost (credit)	0.1		0.1		0.2		0.2
Actuarial loss	2.6		1.5		5.1		2.9
Total	3.1		2.2		6.1		4.5
IPL [Member] | Directly Assigned Defined Benefit Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	1.7		1.8		3.5		3.7
Expected return on plan assets	(2.4)		(2.5)		(4.8)		(4.9)
Amortization of:
Prior service cost (credit)	0		0		(0.1)		(0.1)
Actuarial loss	0.9		0.8		1.9		1.5
Additional benefit costs	0	[1]	2.2	[1]	0	[1]	2.2	[1]
Total	0.2		2.3		0.5		2.4
IPL [Member] | Corporate Service Allocation [Member]
Amortization of:
Additional benefit costs			3.3
IPL [Member] | Cash Balance Plan [Member]
Amortization of:
Additional benefit costs			5.5
WPL [Member] | Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.7		0.7		1.4		1.5
Interest cost	1.1		1.2		2.1		2.6
Expected return on plan assets	(0.4)		(0.4)		(0.7)		(0.7)
Amortization of:
Prior service cost (credit)	(0.9)		(0.8)		(1.9)		(1.1)
Actuarial loss	0.5		0.6		1.1		1.1
Total	1.0		1.3		2.0		3.4
WPL [Member] | Qualified Defined Benefit Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	1.3		1.1		2.6		2.3
Interest cost	4.1		4.0		8.2		8.0
Expected return on plan assets	(5.6)		(5.0)		(11.2)		(10.0)
Amortization of:
Prior service cost (credit)	0.1		0.2		0.2		0.3
Actuarial loss	3.0		1.7		6.1		3.5
Total	2.9		2.0		5.9		4.1
WPL [Member] | Directly Assigned Defined Benefit Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	1.3		1.3		2.6		2.7
Expected return on plan assets	(1.8)		(1.8)		(3.6)		(3.6)
Amortization of:
Prior service cost (credit)	0		0		(0.1)		(0.1)
Actuarial loss	0.9		0.7		1.8		1.4
Additional benefit costs	0	[1]	0.6	[1]	0	[1]	0.6	[1]
Total	0.4		0.8		0.7		1.0
WPL [Member] | Corporate Service Allocation [Member]
Amortization of:
Additional benefit costs			2.4
WPL [Member] | Cash Balance Plan [Member]
Amortization of:
Additional benefit costs			$ 3.0

[1] Alliant Energy reached an agreement with the Internal Revenue Service (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 $8.9 million paid to certain former participants in the Cash Balance Plan in August 2011. Alliant Energy recognized $8.9 million of additional benefits costs in the second quarter of 2011 related to these benefits. IPL was allocated $5.5 million ($2.2 million directly assigned and $3.3 million allocated by Corporate Services) and WPL was allocated $3.0 million ($0.6 million directly assigned and $2.4 million allocated by Corporate Services) of additional benefits costs in the second quarter of 2011 related to these benefits. Refer to Note 11(c) for additional information regarding the Cash Balance Plan.