Summary Of Significant Accounting Policies (Regulatory Assets) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Regulatory Assets [Line Items]
Regulatory Assets	$ 1,495.0	$ 1,141.7
IPL [Member]
Regulatory Assets [Line Items]
Regulatory Assets	1,117.3	799.6
WPL [Member]
Regulatory Assets [Line Items]
Regulatory Assets	377.7	342.1
Tax-Related (Excl. Tax Benefit Rider) [Member]
Regulatory Assets [Line Items]
Regulatory Assets	634.7	395.9
Tax-Related (Excl. Tax Benefit Rider) [Member] | IPL [Member]
Regulatory Assets [Line Items]
Regulatory Assets	614.6	377.2
Tax-Related (Excl. Tax Benefit Rider) [Member] | WPL [Member]
Regulatory Assets [Line Items]
Regulatory Assets	20.1	18.7
Pension And Other Postretirement Benefits Costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets	514.1	418.9
Pension And Other Postretirement Benefits Costs [Member] | IPL [Member]
Regulatory Assets [Line Items]
Regulatory Assets	264.9	217.4
Pension And Other Postretirement Benefits Costs [Member] | WPL [Member]
Regulatory Assets [Line Items]
Regulatory Assets	249.2	201.5
Asset Retirement Obligations (AROs) [Member]
Regulatory Assets [Line Items]
Regulatory Assets	65.9	49.6
Asset Retirement Obligations (AROs) [Member] | IPL [Member]
Regulatory Assets [Line Items]
Regulatory Assets	48.7	33.2
Asset Retirement Obligations (AROs) [Member] | WPL [Member]
Regulatory Assets [Line Items]
Regulatory Assets	17.2	16.4
Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory Assets	77.7	66.8
Derivatives [Member] | IPL [Member]
Regulatory Assets [Line Items]
Regulatory Assets	33.5	24.0
Derivatives [Member] | WPL [Member]
Regulatory Assets [Line Items]
Regulatory Assets	44.2	42.8
Environmental-Related Costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets	38.9	38.4
Environmental-Related Costs [Member] | IPL [Member]
Regulatory Assets [Line Items]
Regulatory Assets	32.2	32.1
Environmental-Related Costs [Member] | WPL [Member]
Regulatory Assets [Line Items]
Regulatory Assets	6.7	6.3
Emission Allowances [Member]
Regulatory Assets [Line Items]
Regulatory Assets	30.0
Emission Allowances [Member] | IPL [Member]
Regulatory Assets [Line Items]
Regulatory Assets	30.0
IPL's Electric Transmission Service Costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets	24.9	33.3
IPL's Electric Transmission Service Costs [Member] | IPL [Member]
Regulatory Assets [Line Items]
Regulatory Assets	24.9	33.3
Proposed Base-Load Projects Costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets	21.5	27.3
Proposed Base-Load Projects Costs [Member] | IPL [Member]
Regulatory Assets [Line Items]
Regulatory Assets	15.3	18.9
Proposed Base-Load Projects Costs [Member] | WPL [Member]
Regulatory Assets [Line Items]
Regulatory Assets	6.2	8.4
Debt Redemption Costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets	21.8	23.7
Debt Redemption Costs [Member] | IPL [Member]
Regulatory Assets [Line Items]
Regulatory Assets	15.1	16.5
Debt Redemption Costs [Member] | WPL [Member]
Regulatory Assets [Line Items]
Regulatory Assets	6.7	7.2
Proposed Clean Air Compliance Projects Costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets	14.9	17.9
Proposed Clean Air Compliance Projects Costs [Member] | IPL [Member]
Regulatory Assets [Line Items]
Regulatory Assets	6.9	9.5
Proposed Clean Air Compliance Projects Costs [Member] | WPL [Member]
Regulatory Assets [Line Items]
Regulatory Assets	8.0	8.4
IPL's Sixth Street Generating Station (Sixth Street) Costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets	13.1	15.7
IPL's Sixth Street Generating Station (Sixth Street) Costs [Member] | IPL [Member]
Regulatory Assets [Line Items]
Regulatory Assets	13.1	15.7
IPL's Flood-Related Costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets	8.0	11.7
IPL's Flood-Related Costs [Member] | IPL [Member]
Regulatory Assets [Line Items]
Regulatory Assets	8.0	11.7
Wholesale Customer Rate Recovery [Member]
Regulatory Assets [Line Items]
Regulatory Assets	3.6	10.5
Wholesale Customer Rate Recovery [Member] | IPL [Member]
Regulatory Assets [Line Items]
Regulatory Assets	0.9	2.6
Wholesale Customer Rate Recovery [Member] | WPL [Member]
Regulatory Assets [Line Items]
Regulatory Assets	2.7	7.9
Other [Member]
Regulatory Assets [Line Items]
Regulatory Assets	25.9	32.0
Other [Member] | IPL [Member]
Regulatory Assets [Line Items]
Regulatory Assets	9.2	7.5
Other [Member] | WPL [Member]
Regulatory Assets [Line Items]
Regulatory Assets	$ 16.7	$ 24.5