Benefit Plans (Defined Benefit Pension And Other Postretirement Benefits Plans) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Alliant Energy Corporation [Member] | Pension Benefits Costs (Credits) [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 3.3	$ 2.9
Interest cost	13.0	13.0
Expected return on plan assets	(17.2)	(16.0)
Amortization of prior service cost (credit)	0.1	0.2
Amortization of actuarial loss	8.3	5.2
Total	7.5	5.3
Alliant Energy Corporation [Member] | Other Postretirement Benefits Costs [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	1.7	2.1
Interest cost	2.6	3.6
Expected return on plan assets	(1.9)	(1.9)
Amortization of prior service cost (credit)	(3.0)	(0.7)
Amortization of actuarial loss	1.6	1.4
Total	1.0	4.5
IPL [Member] | Other Postretirement Benefits Costs [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.8	0.8
Interest cost	1.1	1.7
Expected return on plan assets	(1.3)	(1.3)
Amortization of prior service cost (credit)	(1.6)	(0.3)
Amortization of actuarial loss	0.9	0.8
Total	(0.1)	1.7
IPL [Member] | Qualified Defined Benefit Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	1.9	1.6
Interest cost	4.3	4.2
Expected return on plan assets	(5.8)	(5.0)
Amortization of prior service cost (credit)	0.1	0.1
Amortization of actuarial loss	2.5	1.4
Total	3.0	2.3
IPL [Member] | Directly Assigned Defined Benefit Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	1.8	1.9
Expected return on plan assets	(2.4)	(2.4)
Amortization of prior service cost (credit)	(0.1)	(0.1)
Amortization of actuarial loss	1.0	0.7
Total	0.3	0.1
WPL [Member] | Other Postretirement Benefits Costs [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.7	0.8
Interest cost	1.0	1.4
Expected return on plan assets	(0.3)	(0.3)
Amortization of prior service cost (credit)	(1.0)	(0.3)
Amortization of actuarial loss	0.6	0.5
Total	1.0	2.1
WPL [Member] | Qualified Defined Benefit Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	1.3	1.2
Interest cost	4.1	4.0
Expected return on plan assets	(5.6)	(5.0)
Amortization of prior service cost (credit)	0.1	0.1
Amortization of actuarial loss	3.1	1.8
Total	3.0	2.1
WPL [Member] | Directly Assigned Defined Benefit Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	1.3	1.4
Expected return on plan assets	(1.8)	(1.8)
Amortization of prior service cost (credit)	(0.1)	(0.1)
Amortization of actuarial loss	0.9	0.7
Total	$ 0.3	$ 0.2