Summary Of Significant Accounting Policies (Regulatory Liabilities) (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 906.4	$ 910.1
IPL [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	688.1	721.3
WPL [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	218.3	188.8
Cost of removal obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	406.6	404.9
Cost of removal obligations [Member] | IPL [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	266.5	261.9
Cost of removal obligations [Member] | WPL [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	140.1	143.0
IPL's tax benefit rider [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	327.4	349.6
IPL's tax benefit rider [Member] | IPL [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	327.4	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	48.8	29.6
Energy conservation cost recovery [Member] | IPL [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	8.9	4.7
Energy conservation cost recovery [Member] | WPL [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	39.9	24.9
IPL electric transmission assets sale [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	35.2	45.1
IPL electric transmission assets sale [Member] | IPL [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	35.2	45.1
IPL electric transmission assets sale [Member] | WPL [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	0	0
Derivatives [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	21.3	7.2
Derivatives [Member] | IPL [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	10.2	3.6
Derivatives [Member] | WPL [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	11.1	3.6
Commodity cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	20.4	23.8
Commodity cost recovery [Member] | IPL [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	14.8	23.2
Commodity cost recovery [Member] | WPL [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	5.6	0.6
Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	46.7	49.9
Other [Member] | IPL [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	25.1	33.2
Other [Member] | WPL [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 21.6	$ 16.7