Summary Of Significant Accounting Policies (Regulatory Liabilities) (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 861.6	$ 910.1
IPL [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	655.9	721.3
WPL [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	205.7	188.8
Cost of removal obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	406.8	404.9
Cost of removal obligations [Member] | IPL [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	265.0	261.9
Cost of removal obligations [Member] | WPL [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	141.8	143.0
IPL's tax benefit rider [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	309.1	349.6
IPL's tax benefit rider [Member] | IPL [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	309.1	349.6
IPL's tax benefit rider [Member] | WPL [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	0	0
Energy conservation cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	43.5	29.6
Energy conservation cost recovery [Member] | IPL [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	7.8	4.7
Energy conservation cost recovery [Member] | WPL [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	35.7	24.9
IPL electric transmission assets sale [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	38.0	45.1
IPL electric transmission assets sale [Member] | IPL [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	38.0	45.1
IPL electric transmission assets sale [Member] | WPL [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	0	0
Commodity cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	5.4	23.8
Commodity cost recovery [Member] | IPL [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	0.5	23.2
Commodity cost recovery [Member] | WPL [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	4.9	0.6
Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	58.8	57.1
Other [Member] | IPL [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	35.5	36.8
Other [Member] | WPL [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 23.3	$ 20.3