Summary Of Significant Accounting Policies (Regulatory Liabilities) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 935.7	$ 916.1
Cost of removal obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	411.3	408.7
IPL's tax benefit riders [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	335.1	355.8
Energy conservation cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	61.3	55.1
IPL's electric transmission assets sale [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	29.7	32.5
Commodity cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	39.2	17.7
Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	59.1	46.3
IPL [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	715.2	701.4
IPL [Member] | Cost of removal obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	270.6	268.0
IPL [Member] | IPL's tax benefit riders [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	335.1	355.8
IPL [Member] | Energy conservation cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	16.4	10.0
IPL [Member] | IPL's electric transmission assets sale [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	29.7	32.5
IPL [Member] | Commodity cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	25.4	5.2
IPL [Member] | Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	38.0	29.9
WPL [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	220.5	214.7
WPL [Member] | Cost of removal obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	140.7	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	44.9	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	13.8	12.5
WPL [Member] | Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 21.1	$ 16.4