Summary Of Significant Accounting Policies (Regulatory Assets) (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Regulatory Assets [Line Items]
Regulatory assets	$ 1,479.5	$ 1,495.0
IPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	1,110.4	1,117.3
WPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	369.1	377.7
Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	652.8	634.7
Tax-related [Member] | IPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	630.8	614.6
Tax-related [Member] | WPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	22.0	20.1
Pension and other postretirement benefits costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	502.8	514.1
Pension and other postretirement benefits costs [Member] | IPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	260.0	264.9
Pension and other postretirement benefits costs [Member] | WPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	242.8	249.2
Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	64.2	77.7
Derivatives [Member] | IPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	28.8	33.5
Derivatives [Member] | WPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	35.4	44.2
Asset retirement obligations [Member]
Regulatory Assets [Line Items]
Regulatory assets	58.0	65.9
Asset retirement obligations [Member] | IPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	39.6	48.7
Asset retirement obligations [Member] | WPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	18.4	17.2
Environmental-related costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	37.1	38.9
Environmental-related costs [Member] | IPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	32.2	32.2
Environmental-related costs [Member] | WPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	4.9	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
Emission allowances [Member] | WPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	0	0
IPL's electric transmission service costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	20.8	24.9
IPL's electric transmission service costs [Member] | IPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	20.8	24.9
IPL's electric transmission service costs [Member] | WPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	0	0
Debt redemption costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	20.8	21.8
Debt redemption costs [Member] | IPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	14.3	15.1
Debt redemption costs [Member] | WPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	6.5	6.7
Proposed base-load projects costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	17.8	21.5
Proposed base-load projects costs [Member] | IPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	12.7	15.3
Proposed base-load projects costs [Member] | WPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	5.1	6.2
Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	75.2	65.5
Other [Member] | IPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	41.2	38.1
Other [Member] | WPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	$ 34.0	$ 27.4