Summary Of Significant Accounting Policies (Regulatory Liabilities) (Details) (USD $) In Millions	Jun. 30, 2011	Dec. 31, 2010
Regulatory Liabilities	$ 940.1	$ 800.1
IPL [Member] | Cost Of Removal Obligations [Member]
Regulatory Liabilities	258.8	257.6
WPL [Member] | Cost Of Removal Obligations [Member]
Regulatory Liabilities	143.1	137.8
Cost Of Removal Obligations [Member]
Regulatory Liabilities	401.9	395.4
IPL [Member] | IPL's Customer Cost Management Plan [Member]
Regulatory Liabilities	341.3	193.5
IPL's Customer Cost Management Plan [Member]
Regulatory Liabilities	341.3	193.5
IPL [Member] | IPL Electric Transmission Assets Sale [Member]
Regulatory Liabilities	59.9	71.8
IPL Electric Transmission Assets Sale [Member]
Regulatory Liabilities	59.9	71.8
IPL [Member] | Emission Allowances [Member]
Regulatory Liabilities	28.3	33.9
WPL [Member] | Emission Allowances [Member]
Regulatory Liabilities		0.5
Emission Allowances [Member]
Regulatory Liabilities	28.3	34.4
IPL [Member] | Commodity Cost Recovery [Member]
Regulatory Liabilities	20.0	7.5
WPL [Member] | Commodity Cost Recovery [Member]
Regulatory Liabilities	3.1	5.2
Commodity Cost Recovery [Member]
Regulatory Liabilities	23.1	12.7
IPL [Member] | Energy Conservation Cost Recovery [Member]
Regulatory Liabilities	2.5	1.7
WPL [Member] | Energy Conservation Cost Recovery [Member]
Regulatory Liabilities	17.9	6.9
Energy Conservation Cost Recovery [Member]
Regulatory Liabilities	20.4	8.6
IPL [Member] | Tax-Related [Member]
Regulatory Liabilities	3.8	4.6
WPL [Member] | Tax-Related [Member]
Regulatory Liabilities	11.2	12.2
Tax-Related [Member]
Regulatory Liabilities	15.0	16.8
IPL [Member] | Other [Member]
Regulatory Liabilities	23.6	15.0
WPL [Member] | Other [Member]
Regulatory Liabilities	9.5	9.6
Other [Member]
Regulatory Liabilities	33.1	24.6
IPL [Member] | IPL's Duane Arnold Energy Center (DAEC) sale[Member]
Regulatory Liabilities	17.1	42.3
IPL's Duane Arnold Energy Center (DAEC) sale[Member]
Regulatory Liabilities	17.1	42.3
IPL [Member]
Regulatory Liabilities	755.3	627.9
WPL [Member]
Regulatory Liabilities	$ 184.8	$ 172.2