Summary Of Significant Accounting Policies (Regulatory Assets) (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Regulatory Assets [Line Items]
Regulatory assets	$ 1,518.4	$ 1,495.0
IPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	1,151.0	1,117.3
WPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	367.4	377.7
Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	709.4	634.7
Tax-related [Member] | IPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	686.2	614.6
Tax-related [Member] | WPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	23.2	20.1
Pension and other postretirement benefits costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	495.4	514.1
Pension and other postretirement benefits costs [Member] | IPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	256.3	264.9
Pension and other postretirement benefits costs [Member] | WPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	239.1	249.2
Asset retirement obligations [Member]
Regulatory Assets [Line Items]
Regulatory assets	57.8	65.9
Asset retirement obligations [Member] | IPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	38.7	48.7
Asset retirement obligations [Member] | WPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	19.1	17.2
Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	56.6	77.7
Derivatives [Member] | IPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	22.6	33.5
Derivatives [Member] | WPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	34.0	44.2
Environmental-related costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	36.3	38.9
Environmental-related costs [Member] | IPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	31.5	32.2
Environmental-related costs [Member] | WPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	4.8	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
Debt redemption costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	20.3	21.8
Debt redemption costs [Member] | IPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	14.0	15.1
Debt redemption costs [Member] | WPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	6.3	6.7
IPL's electric transmission service costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	18.7	24.9
IPL's electric transmission service costs [Member] | IPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	18.7	24.9
IPL's electric transmission service costs [Member] | WPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	0	0
Proposed base-load projects costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	16.0	21.5
Proposed base-load projects costs [Member] | IPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	11.4	15.3
Proposed base-load projects costs [Member] | WPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	4.6	6.2
Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	77.9	65.5
Other [Member] | IPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	41.6	38.1
Other [Member] | WPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	$ 36.3	$ 27.4