Summary Of Significant Accounting Policies (Regulatory Assets) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Regulatory Assets [Line Items]
Regulatory assets	$ 1,612.4	$ 1,495.0
IPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	1,217.9	1,117.3
WPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	394.5	377.7
Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	770.7	634.7
Tax-related [Member] | IPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	746.2	614.6
Tax-related [Member] | WPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	24.5	20.1
Pension and other postretirement benefits costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	549.2	514.1
Pension and other postretirement benefits costs [Member] | IPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	279.3	264.9
Pension and other postretirement benefits costs [Member] | WPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	269.9	249.2
Asset retirement obligations [Member]
Regulatory Assets [Line Items]
Regulatory assets	62.4	65.9
Asset retirement obligations [Member] | IPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	38.6	48.7
Asset retirement obligations [Member] | WPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	23.8	17.2
Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	40.2	77.7
Derivatives [Member] | IPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	16.3	33.5
Derivatives [Member] | WPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	23.9	44.2
Environmental-related costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	34.9	38.9
Environmental-related costs [Member] | IPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	30.3	32.2
Environmental-related costs [Member] | WPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	4.6	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	19.8	21.8
Debt redemption costs [Member] | IPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	13.6	15.1
Debt redemption costs [Member] | WPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	6.2	6.7
Proposed clean air compliance projects costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	16.7	14.9
Proposed clean air compliance projects costs [Member] | IPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	7.8	6.9
Proposed clean air compliance projects costs [Member] | WPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	8.9	8.0
IPL's electric transmission service costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	16.6	24.9
IPL's electric transmission service costs [Member] | IPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	16.6	24.9
IPL's electric transmission service costs [Member] | WPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	0	0
Proposed coal-fired base-load projects costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	14.2	21.5
Proposed coal-fired base-load projects costs [Member] | IPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	10.1	15.3
Proposed coal-fired base-load projects costs [Member] | WPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	4.1	6.2
Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	57.7	50.6
Other [Member] | IPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	29.1	31.2
Other [Member] | WPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	$ 28.6	$ 19.4