Summary Of Significant Accounting Policies (Regulatory Assets) (Details) (USD $) In Millions	Sep. 30, 2011	Dec. 31, 2010
Regulatory Assets	$ 1,295.7	$ 1,141.7
IPL [Member] | Tax-Related [Member]
Regulatory Assets	589.1	377.2
WPL [Member] | Tax-Related [Member]
Regulatory Assets	19.7	18.7
Tax-Related [Member]
Regulatory Assets	608.8	395.9
IPL [Member] | Pension And Other Postretirement Benefits [Member]
Regulatory Assets	174.7	217.4
WPL [Member] | Pension And Other Postretirement Benefits [Member]
Regulatory Assets	176.1	201.5
Pension And Other Postretirement Benefits [Member]
Regulatory Assets	350.8	418.9
IPL [Member] | Asset Retirement Obligations [Member]
Regulatory Assets	48.6	33.2
WPL [Member] | Asset Retirement Obligations [Member]
Regulatory Assets	16.6	16.4
Asset Retirement Obligations [Member]
Regulatory Assets	65.2	49.6
IPL [Member] | Derivatives [Member]
Regulatory Assets	20.6	24.0
WPL [Member] | Derivatives [Member]
Regulatory Assets	31.5	42.8
Derivatives [Member]
Regulatory Assets	52.1	66.8
IPL [Member] | Environmental-Related Costs [Member]
Regulatory Assets	32.6	32.1
WPL [Member] | Environmental-Related Costs [Member]
Regulatory Assets	6.8	6.3
Environmental-Related Costs [Member]
Regulatory Assets	39.4	38.4
IPL [Member] | Emission Allowances [Member]
Regulatory Assets	30.0
Emission Allowances [Member]
Regulatory Assets	30.0
IPL [Member] | IPL's Electric Transmission Service Costs [Member]
Regulatory Assets	27.0	33.3
IPL's Electric Transmission Service Costs [Member]
Regulatory Assets	27.0	33.3
IPL [Member] | Proposed Base-Load Projects [Member]
Regulatory Assets	16.6	18.9
WPL [Member] | Proposed Base-Load Projects [Member]
Regulatory Assets	6.7	8.4
Proposed Base-Load Projects [Member]
Regulatory Assets	23.3	27.3
IPL [Member] | Debt Redemption Costs [Member]
Regulatory Assets	15.4	16.5
WPL [Member] | Debt Redemption Costs [Member]
Regulatory Assets	6.8	7.2
Debt Redemption Costs [Member]
Regulatory Assets	22.2	23.7
IPL [Member] | Other [Member]
Regulatory Assets	48.2	47.0
WPL [Member] | Other [Member]
Regulatory Assets	28.7	40.8
Other [Member]
Regulatory Assets	76.9	87.8
IPL [Member]
Regulatory Assets	1,002.8	799.6
WPL [Member]
Regulatory Assets	$ 292.9	$ 342.1