Summary Of Significant Accounting Policies (Regulatory Liabilities) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Regulatory Liabilities [Line Items]
Regulatory Liabilities	$ 882.5	$ 910.1
IPL [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	687.1	721.3
WPL [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	195.4	188.8
Cost Of Removal Obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	407.3	404.9
Cost Of Removal Obligations [Member] | IPL [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	264.7	261.9
Cost Of Removal Obligations [Member] | WPL [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	142.6	143.0
IPL's Tax Benefit Rider [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	329.4	349.6
IPL's Tax Benefit Rider [Member] | IPL [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	329.4	349.6
IPL's Electric Transmission Assets Sale [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	42.8	45.1
IPL's Electric Transmission Assets Sale [Member] | IPL [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	42.8	45.1
Energy Conservation Cost Recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	37.2	29.6
Energy Conservation Cost Recovery [Member] | IPL [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	7.0	4.7
Energy Conservation Cost Recovery [Member] | WPL [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	30.2	24.9
Commodity Cost Recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	10.5	23.8
Commodity Cost Recovery [Member] | IPL [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	8.6	23.2
Commodity Cost Recovery [Member] | WPL [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	1.9	0.6
Other [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	55.3	57.1
Other [Member] | IPL [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	34.6	36.8
Other [Member] | WPL [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	$ 20.7	$ 20.3