Summary Of Significant Accounting Policies (Regulatory Assets) (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Regulatory Assets [Line Items]
Regulatory assets	$ 1,612.1	$ 1,612.4
Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	793.1	770.7
Pension and other postretirement benefits costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	533.2	549.2
Asset retirement obligations [Member]
Regulatory Assets [Line Items]
Regulatory assets	66.6	62.4
Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	37.1	40.2
Environmental-related costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	31.5	34.9
Emission allowances [Member]
Regulatory Assets [Line Items]
Regulatory assets	30.0	30.0
Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	120.6	125.0
IPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	1,224.2	1,217.9
IPL [Member] | Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	768.2	746.2
IPL [Member] | Pension and other postretirement benefits costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	271.7	279.3
IPL [Member] | Asset retirement obligations [Member]
Regulatory Assets [Line Items]
Regulatory assets	39.2	38.6
IPL [Member] | Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	10.0	16.3
IPL [Member] | Environmental-related costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	27.2	30.3
IPL [Member] | Emission allowances [Member]
Regulatory Assets [Line Items]
Regulatory assets	30.0	30.0
IPL [Member] | Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	77.9	77.2
WPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	387.9	394.5
WPL [Member] | Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	24.9	24.5
WPL [Member] | Pension and other postretirement benefits costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	261.5	269.9
WPL [Member] | Asset retirement obligations [Member]
Regulatory Assets [Line Items]
Regulatory assets	27.4	23.8
WPL [Member] | Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	27.1	23.9
WPL [Member] | Environmental-related costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	4.3	4.6
WPL [Member] | Emission allowances [Member]
Regulatory Assets [Line Items]
Regulatory assets	0	0
WPL [Member] | Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	$ 42.7	$ 47.8