Summary Of Significant Accounting Policies (Regulatory Assets) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Regulatory Assets [Line Items]
Regulatory assets	$ 1,489.4	$ 1,495.0
IPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	1,112.9	1,117.3
WPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	376.5	377.7
Tax-Related [Member]
Regulatory Assets [Line Items]
Regulatory assets	639.2	634.7
Tax-Related [Member] | IPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	618.2	614.6
Tax-Related [Member] | WPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	21.0	20.1
Pension And Other Postretirement Benefits Costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	510.5	514.1
Pension And Other Postretirement Benefits Costs [Member] | IPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	263.6	264.9
Pension And Other Postretirement Benefits Costs [Member] | WPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	246.9	249.2
Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	83.1	77.7
Derivatives [Member] | IPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	38.1	33.5
Derivatives [Member] | WPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	45.0	44.2
Asset Retirement Obligations [Member]
Regulatory Assets [Line Items]
Regulatory assets	57.4	65.9
Asset Retirement Obligations [Member] | IPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	39.7	48.7
Asset Retirement Obligations [Member] | WPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	17.7	17.2
Environmental-Related Costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	37.6	38.9
Environmental-Related Costs [Member] | IPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	32.6	32.2
Environmental-Related Costs [Member] | WPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	5.0	6.7
Emission Allowances [Member]
Regulatory Assets [Line Items]
Regulatory assets	30.0	30.0
Emission Allowances [Member] | IPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	30.0	30.0
IPL's Electric Transmission Service Costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	22.9	24.9
IPL's Electric Transmission Service Costs [Member] | IPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	22.9	24.9
Debt Redemption Costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	21.3	21.8
Debt Redemption Costs [Member] | IPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	14.7	15.1
Debt Redemption Costs [Member] | WPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	6.6	6.7
Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	87.4	87.0
Other [Member] | IPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	53.1	53.4
Other [Member] | WPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	$ 34.3	$ 33.6