Benefit Plans (Funded Status Of Benefits Plans - Directly Allocated Defined Benefit Pension Plans) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011
Amounts recognized on the Consolidated Balance Sheets consist of:
Pension and other benefit obligations	$ (364.0)		$ (312.7)
IPL [Member]
Amounts recognized on the Consolidated Balance Sheets consist of:
Pension and other benefit obligations	(122.9)		(101.9)
IPL [Member] | Directly Assigned Defined Benefit Pension Benefits [Member]
Change in projected benefit obligation:
Net projected benefit obligation at January 1	155.4		144.4
Interest cost	7.0		7.3
Plan amendments	0		2.8
Actuarial loss	17.9		13.2
Gross benefits paid	(9.5)		(12.3)
Net projected benefit obligation at December 31	170.8		155.4
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at January 1	127.4		127.6
Actual return on plan assets	17.9		4.2
Employer contributions	0.7		7.9
Gross benefits paid	(9.5)		(12.3)
Fair value of plan assets at December 31	136.5		127.4
Under funded status at December 31	(34.3)		(28.0)
Amounts recognized on the Consolidated Balance Sheets consist of:
Other current liabilities	(0.8)		(0.8)
Pension and other benefit obligations	(33.5)		(27.2)
Net amount recognized at December 31	(34.3)		(28.0)
Amounts recognized in Regulatory Assets:
Net actuarial loss	81.9	[1]	76.2	[1]
Prior service credit	(3.2)	[1]	(3.4)	[1]
Net amount recognized at December 31	78.7	[1]	72.8	[1]
Accumulated benefit obligations	170.8		155.4
WPL [Member]
Amounts recognized on the Consolidated Balance Sheets consist of:
Pension and other benefit obligations	(159.7)		(128.0)
WPL [Member] | Directly Assigned Defined Benefit Pension Benefits [Member]
Change in projected benefit obligation:
Net projected benefit obligation at January 1	115.2		107.4
Interest cost	5.2		5.5
Plan amendments	0		0.7
Actuarial loss	14.5		10.3
Gross benefits paid	(8.4)		(8.7)
Net projected benefit obligation at December 31	126.5		115.2
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at January 1	97.0		95.6
Actual return on plan assets	13.5		3.2
Employer contributions	0.1		6.9
Gross benefits paid	(8.4)		(8.7)
Fair value of plan assets at December 31	102.2		97.0
Under funded status at December 31	(24.3)		(18.2)
Amounts recognized on the Consolidated Balance Sheets consist of:
Other current liabilities	(0.2)		(0.2)
Pension and other benefit obligations	(24.1)		(18.0)
Net amount recognized at December 31	(24.3)		(18.2)
Amounts recognized in Regulatory Assets:
Net actuarial loss	78.4	[1]	73.7	[1]
Prior service credit	(2.3)	[1]	(2.4)	[1]
Net amount recognized at December 31	76.1	[1]	71.3	[1]
Accumulated benefit obligations	$ 126.5		$ 115.2

[1] Refer to Note 1(b) for amounts recognized in “Regulatory assets” on IPL’s and WPL’s Consolidated Balance Sheets.