Fair Value Measurements (Fair Value Measurements Using Significant Unobservable Inputs) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012		Mar. 31, 2011
Commodity Contracts [Member]
Beginning balance, Jan. 1	$ (0.9)		$ 2.8
Total losses (realized/unrealized) included in changes in net assets	(12.5)	[1]	(0.5)	[1]
Transfers into Level 3	(3.8)	[2]
Transfers out of Level 3	5.3	[3]
Settlements	(3.2)	[4]	(3.4)	[4]
Ending balance, March 31	(15.1)		(1.1)
The amount of total losses for the period included in changes in net assets attributable to the change in unrealized losses relating to assets and liabilities held at March 31	(12.5)		(0.5)
Commodity Contracts [Member] | IPL [Member]
Beginning balance, Jan. 1	4.3		4.3
Total losses (realized/unrealized) included in changes in net assets	(10.8)	[1]	(0.2)	[1]
Transfers into Level 3	(2.7)	[2]
Transfers out of Level 3	0.1	[3]
Settlements	(2.3)	[4]	(2.7)	[4]
Ending balance, March 31	(11.4)		1.4
The amount of total losses for the period included in changes in net assets attributable to the change in unrealized losses relating to assets and liabilities held at March 31	(10.8)		(0.2)
Commodity Contracts [Member] | WPL [Member]
Beginning balance, Jan. 1	(5.2)		(1.5)
Total losses (realized/unrealized) included in changes in net assets	(1.7)	[1]	(0.3)	[1]
Transfers into Level 3	(1.1)	[2]
Transfers out of Level 3	5.2	[3]
Settlements	(0.9)		(0.7)
Ending balance, March 31	(3.7)		(2.5)
The amount of total losses for the period included in changes in net assets attributable to the change in unrealized losses relating to assets and liabilities held at March 31	(1.7)	[1]	(0.3)	[1]
Foreign Contracts [Member]
Beginning balance, Jan. 1			4.7
Settlements			(2.6)	[4]
Ending balance, March 31			2.1
Foreign Contracts [Member] | IPL [Member]
Beginning balance, Jan. 1			4.8
Settlements			(2.7)	[4]
Ending balance, March 31			2.1
Foreign Contracts [Member] | WPL [Member]
Beginning balance, Jan. 1			(0.1)
Settlements			0.1
Deferred Proceeds [Member]
Beginning balance, Jan. 1	53.7		152.9
Settlements	(20.8)	[4]	(23.1)	[4]
Ending balance, March 31	32.9		129.8
Deferred Proceeds [Member] | IPL [Member]
Beginning balance, Jan. 1	53.7		152.9
Settlements	(20.8)	[4]	(23.1)	[4]
Ending balance, March 31	$ 32.9		$ 129.8

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

[2] Markets for similar assets and liabilities were not active and observable market inputs were not available for transfers into Level 3. The transfers were valued as of the beginning of the period.

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

[4] 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.