Summary Of Significant Accounting Policies (Regulatory Liabilities) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Regulatory Liabilities [Line Items]
Regulatory Liabilities	$ 910.1	$ 800.1
IPL [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	721.3	627.9
WPL [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	188.8	172.2
Cost Of Removal Obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	404.9	395.4
Cost Of Removal Obligations [Member] | IPL [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	261.9	257.6
Cost Of Removal Obligations [Member] | WPL [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	143.0	137.8
IPL's Tax Benefit Rider [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	349.6	193.5
IPL's Tax Benefit Rider [Member] | IPL [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	349.6	193.5
IPL Electric Transmission Assets Sale [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	45.1	71.8
IPL Electric Transmission Assets Sale [Member] | IPL [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	45.1	71.8
Energy Conservation Cost Recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	29.6	8.6
Energy Conservation Cost Recovery [Member] | IPL [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	4.7	1.7
Energy Conservation Cost Recovery [Member] | WPL [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	24.9	6.9
Commodity Cost Recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	23.8	12.7
Commodity Cost Recovery [Member] | IPL [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	23.2	7.5
Commodity Cost Recovery [Member] | WPL [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	0.6	5.2
IPL's Duane Arnold Energy Center (DAEC) Sale[Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	14.6	42.3
IPL's Duane Arnold Energy Center (DAEC) Sale[Member] | IPL [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	14.6	42.3
Emission Allowances [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities		34.4
Emission Allowances [Member] | IPL [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities		33.9
Emission Allowances [Member] | WPL [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities		0.5
Other [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	42.5	41.4
Other [Member] | IPL [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	22.2	19.6
Other [Member] | WPL [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	$ 20.3	$ 21.8