Fair Value Measurements (Fair Value Measurements Using Significant Unobservable Inputs) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010
Commodity Contracts [Member]
Beginning balance	$ 2.8		$ 2.6
Total gains (losses) (realized/unrealized) included in changes in net assets	(7.3)	[1]	(0.8)	[1]
Transfers in and/or out of Level 3	0.2	[2]	(0.3)	[2]
Purchases	21.8
Settlements	(18.4)	[3]	1.3	[3]
Ending balance	(0.9)		2.8
The amount of total gains (losses) for the period included in changes in net assets attributable to the change in unrealized gains (losses) relating to assets and liabilities held at Dec. 31 (a)	(7.3)	[1]	(0.8)	[1]
Commodity Contracts [Member] | IPL [Member]
Beginning balance	4.3		1.4
Total gains (losses) (realized/unrealized) included in changes in net assets	(3.1)	[1]	0.5	[1]
Purchases	18.1
Settlements	(15.0)	[3]	2.4	[3]
Ending balance	4.3		4.3
The amount of total gains (losses) for the period included in changes in net assets attributable to the change in unrealized gains (losses) relating to assets and liabilities held at Dec. 31 (a)	(3.1)	[1]	0.5	[1]
Commodity Contracts [Member] | WPL [Member]
Beginning balance	(1.5)		1.2
Total gains (losses) (realized/unrealized) included in changes in net assets	(4.2)	[1]	(1.3)	[1]
Transfers in and/or out of Level 3	0.2	[2]	(0.3)	[2]
Purchases	3.7
Settlements	(3.4)		(1.1)
Ending balance	(5.2)		(1.5)
The amount of total gains (losses) for the period included in changes in net assets attributable to the change in unrealized gains (losses) relating to assets and liabilities held at Dec. 31 (a)	(4.2)	[1]	(1.3)	[1]
Foreign Contracts [Member]
Beginning balance	4.7		3.1
Total gains (losses) (realized/unrealized) included in changes in net assets			3.8	[1]
Settlements	(4.7)	[3]	(2.2)	[3]
Ending balance			4.7
The amount of total gains (losses) for the period included in changes in net assets attributable to the change in unrealized gains (losses) relating to assets and liabilities held at Dec. 31 (a)			3.8	[1]
Foreign Contracts [Member] | IPL [Member]
Beginning balance	4.8		4.6
Total gains (losses) (realized/unrealized) included in changes in net assets		[1]	3.8	[1]
Purchases
Sales	(2.1)	[4]
Settlements	(2.7)	[3]	(3.6)	[3]
Ending balance			4.8
The amount of total gains (losses) for the period included in changes in net assets attributable to the change in unrealized gains (losses) relating to assets and liabilities held at Dec. 31 (a)		[1]	3.8	[1]
Foreign Contracts [Member] | WPL [Member]
Beginning balance	(0.1)		(1.5)
Total gains (losses) (realized/unrealized) included in changes in net assets		[1]
Transfers in and/or out of Level 3		[2]
Purchases
Settlements	0.1		1.4
Ending balance			(0.1)
The amount of total gains (losses) for the period included in changes in net assets attributable to the change in unrealized gains (losses) relating to assets and liabilities held at Dec. 31 (a)		[1]
Deferred Proceeds [Member]
Beginning balance	152.9
Settlements	(99.2)	[3]	152.9	[3]
Ending balance	53.7		152.9
Deferred Proceeds [Member] | IPL [Member]
Beginning balance	152.9
Settlements	(99.2)	[3]	152.9	[3]
Ending balance	$ 53.7		$ 152.9

[1] Gains and losses related to derivative assets and derivative liabilities are recorded in "Regulatory assets" and "Regulatory liabilities" on the Consolidated Balance Sheets.

[2] Observable market inputs became available for certain commodity contracts previously classified as Level 3. The transfers were valued as of the beginning of the period.

[3] Settlements related to deferred proceeds are due to the change in the carrying amount of receivables sold less the allowance for doubtful accounts associated with the receivables sold and cash proceeds received from the receivables sold.

[4] The foreign exchange contract was transferred from IPL to Resources in connection with the sale of wind project assets in 2011.