Summary Of Significant Accounting Policies (Regulatory Assets) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Regulatory Assets [Line Items]
Regulatory assets	$ 1,582.6	$ 1,612.4
Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	781.9	770.7
Pension and other postretirement benefits costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	541.4	549.2
Asset retirement obligations [Member]
Regulatory Assets [Line Items]
Regulatory assets	62.9	62.4
Environmental-related costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	32.2	34.9
Emission allowances [Member]
Regulatory Assets [Line Items]
Regulatory assets	30.0	30.0
Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	17.0	40.2
Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	117.2	125.0
IPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	1,207.9	1,217.9
IPL [Member] | Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	757.2	746.2
IPL [Member] | Pension and other postretirement benefits costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	275.6	279.3
IPL [Member] | Asset retirement obligations [Member]
Regulatory Assets [Line Items]
Regulatory assets	38.5	38.6
IPL [Member] | Environmental-related costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	27.7	30.3
IPL [Member] | Emission allowances [Member]
Regulatory Assets [Line Items]
Regulatory assets	30.0	30.0
IPL [Member] | Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	6.3	16.3
IPL [Member] | Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	72.6	77.2
WPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	374.7	394.5
WPL [Member] | Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	24.7	24.5
WPL [Member] | Pension and other postretirement benefits costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	265.8	269.9
WPL [Member] | Asset retirement obligations [Member]
Regulatory Assets [Line Items]
Regulatory assets	24.4	23.8
WPL [Member] | Environmental-related costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	4.5	4.6
WPL [Member] | Emission allowances [Member]
Regulatory Assets [Line Items]
Regulatory assets	0	0
WPL [Member] | Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	10.7	23.9
WPL [Member] | Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	$ 44.6	$ 47.8