Summary Of Significant Accounting Policies (Regulatory Liabilities) (Details) (USD $) In Millions	Sep. 30, 2011	Dec. 31, 2010
Regulatory Liabilities	$ 904.0	$ 800.1
IPL [Member] | Cost Of Removal Obligations [Member]
Regulatory Liabilities	260.0	257.6
WPL [Member] | Cost Of Removal Obligations [Member]
Regulatory Liabilities	143.0	137.8
Cost Of Removal Obligations [Member]
Regulatory Liabilities	403.0	395.4
IPL [Member] | IPL's Tax Benefit Rider [Member]
Regulatory Liabilities	344.3	193.5
IPL's Tax Benefit Rider [Member]
Regulatory Liabilities	344.3	193.5
IPL [Member] | IPL Electric Transmission Assets Sale [Member]
Regulatory Liabilities	51.7	71.8
IPL Electric Transmission Assets Sale [Member]
Regulatory Liabilities	51.7	71.8
IPL [Member] | Energy Conservation Cost Recovery [Member]
Regulatory Liabilities	1.7	1.7
WPL [Member] | Energy Conservation Cost Recovery [Member]
Regulatory Liabilities	20.7	6.9
Energy Conservation Cost Recovery [Member]
Regulatory Liabilities	22.4	8.6
IPL [Member] | Commodity Cost Recovery [Member]
Regulatory Liabilities	15.9	7.5
WPL [Member] | Commodity Cost Recovery [Member]
Regulatory Liabilities	2.2	5.2
Commodity Cost Recovery [Member]
Regulatory Liabilities	18.1	12.7
IPL [Member] | IPL's Duane Arnold Energy Center (DAEC) Sale[Member]
Regulatory Liabilities	15.9	42.3
IPL's Duane Arnold Energy Center (DAEC) Sale[Member]
Regulatory Liabilities	15.9	42.3
IPL [Member] | Tax-Related [Member]
Regulatory Liabilities	3.6	4.6
WPL [Member] | Tax-Related [Member]
Regulatory Liabilities	11.0	12.2
Tax-Related [Member]
Regulatory Liabilities	14.6	16.8
IPL [Member] | Emission Allowances [Member]
Regulatory Liabilities	1.9	33.9
WPL [Member] | Emission Allowances [Member]
Regulatory Liabilities		0.5
Emission Allowances [Member]
Regulatory Liabilities	1.9	34.4
IPL [Member] | Other [Member]
Regulatory Liabilities	18.8	15.0
WPL [Member] | Other [Member]
Regulatory Liabilities	13.3	9.6
Other [Member]
Regulatory Liabilities	32.1	24.6
IPL [Member]
Regulatory Liabilities	713.8	627.9
WPL [Member]
Regulatory Liabilities	$ 190.2	$ 172.2