Summary Of Significant Accounting Policies (Regulatory Liabilities) (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 837.6	$ 916.1
Cost of removal obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	415.7	408.7
IPL's tax benefit riders [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	288.8	355.8
Energy conservation cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	58.1	55.1
IPL's electric transmission assets sale [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	24.3	32.5
Commodity cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	7.0	17.7
Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	43.7	46.3
IPL [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	630.4	701.4
IPL [Member] | Cost of removal obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	275.8	268.0
IPL [Member] | IPL's tax benefit riders [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	288.8	355.8
IPL [Member] | Energy conservation cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	12.6	10.0
IPL [Member] | IPL's electric transmission assets sale [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	24.3	32.5
IPL [Member] | Commodity cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	1.1	5.2
IPL [Member] | Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	27.8	29.9
WPL [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	207.2	214.7
WPL [Member] | Cost of removal obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	139.9	140.7
WPL [Member] | IPL's tax benefit riders [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	0	0
WPL [Member] | Energy conservation cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	45.5	45.1
WPL [Member] | IPL's electric transmission assets sale [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	0	0
WPL [Member] | Commodity cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	5.9	12.5
WPL [Member] | Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 15.9	$ 16.4