Fair Value Measurements (Fair Value Measurements Using Significant Unobservable Inputs) (Details) (USD $) In Millions	3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended				3 Months Ended		6 Months Ended								3 Months Ended				6 Months Ended				3 Months Ended	6 Months Ended		3 Months Ended				6 Months Ended			3 Months Ended		6 Months Ended			3 Months Ended				6 Months Ended
	Jun. 30, 2011 IPL [Member] Commodity Contracts [Member]	Jun. 30, 2010 IPL [Member] Commodity Contracts [Member]	Jun. 30, 2011 IPL [Member] Commodity Contracts [Member]	Jun. 30, 2010 IPL [Member] Commodity Contracts [Member]	Jun. 30, 2011 IPL [Member] Foreign Contracts [Member]		Jun. 30, 2011 IPL [Member] Foreign Contracts [Member]		Jun. 30, 2010 IPL [Member] Foreign Contracts [Member]	Mar. 31, 2010 IPL [Member] Foreign Contracts [Member]	Jun. 30, 2011 IPL [Member] Deferred Proceeds [Member]	Jun. 30, 2010 IPL [Member] Deferred Proceeds [Member]	Jun. 30, 2011 IPL [Member] Deferred Proceeds [Member]	Jun. 30, 2010 IPL [Member] Deferred Proceeds [Member]	Jun. 30, 2011 IPL [Member] Available-for-Sale Securities [Member]	Mar. 31, 2011 IPL [Member] Available-for-Sale Securities [Member]	Dec. 31, 2010 IPL [Member] Available-for-Sale Securities [Member]	Jun. 30, 2010 IPL [Member] Available-for-Sale Securities [Member]	Mar. 31, 2010 IPL [Member] Available-for-Sale Securities [Member]	Dec. 31, 2009 IPL [Member] Available-for-Sale Securities [Member]	Jun. 30, 2011 WPL [Member] Commodity Contracts [Member]		Jun. 30, 2010 WPL [Member] Commodity Contracts [Member]		Jun. 30, 2011 WPL [Member] Commodity Contracts [Member]		Jun. 30, 2010 WPL [Member] Commodity Contracts [Member]		Jun. 30, 2010 WPL [Member] Foreign Contracts [Member]	Jun. 30, 2011 WPL [Member] Foreign Contracts [Member]	Jun. 30, 2010 WPL [Member] Foreign Contracts [Member]	Jun. 30, 2011 Commodity Contracts [Member]		Jun. 30, 2010 Commodity Contracts [Member]		Jun. 30, 2011 Commodity Contracts [Member]		Jun. 30, 2010 Commodity Contracts [Member]	Jun. 30, 2010 Foreign Contracts [Member]		Jun. 30, 2011 Foreign Contracts [Member]	Jun. 30, 2010 Foreign Contracts [Member]	Mar. 31, 2011 Foreign Contracts [Member]	Jun. 30, 2011 Deferred Proceeds [Member]		Jun. 30, 2010 Deferred Proceeds [Member]		Jun. 30, 2011 Deferred Proceeds [Member]	Jun. 30, 2010 Deferred Proceeds [Member]	Jun. 30, 2011 Available-for-Sale Securities [Member]	Mar. 31, 2011 Available-for-Sale Securities [Member]	Dec. 31, 2010 Available-for-Sale Securities [Member]	Jun. 30, 2010 Available-for-Sale Securities [Member]	Mar. 31, 2010 Available-for-Sale Securities [Member]	Dec. 31, 2009 Available-for-Sale Securities [Member]
Beginning balance	$ 1.4	$ 0.6	$ 4.3	$ 1.4	$ 2.1		$ 4.8		$ 4.6	$ 8.4	$ 129.8		$ 152.9		$ 0.1	$ 0.1	$ 0.1	$ 0.1	$ 0.1	$ 0.1	$ (2.5)		$ (2.4)		$ (1.5)		$ 1.2		$ (1.1)	$ (0.1)	$ (1.5)	$ (1.1)		$ (1.8)		$ 2.8		$ 2.6	$ 7.3		$ 4.7	$ 3.1	$ 2.1	$ 129.8				$ 152.9		$ 0.1	$ 0.1	$ 0.1	$ 0.1	$ 0.1	$ 0.1
Total gains or (losses) (realized/unrealized) included in changes in net assets	0.7	10.7	0.5	10.6					3.8												(0.2)	[1]	3.2	[1]	(0.4)	[1]	1.6	[1]				0.5	[1]	13.9	[1]	0.1		12.2				3.8
Transfers in and/or out of Level 3																					0.3	[2]	0.6	[2]	0.2	[2]	(0.1)	[2]				0.3	[2]	0.6	[2]	0.2		(0.1)
Purchases	18.1		18.1																		3.7				3.7							21.8				21.8	[1]
Sales					(2.1)	[1]	(2.1)	[1]
Settlements	(2.7)	(1.6)	(5.4)	(2.3)			(2.7)				(63.4)	68.1	(86.5)	68.1							(0.7)		(0.8)		(1.4)		(2.1)		0.6	0.1	1.0	(3.4)	[3]	(2.4)	[3]	(6.8)		(4.4)	0.6	[3]	(2.6)	1.0		(63.4)	[3]	68.1	[3]	(86.5)	68.1
Ending balance	17.5	9.7	17.5	9.7					8.4	8.4	66.4	68.1	66.4	68.1	0.1	0.1	0.1	0.1	0.1	0.1	0.6		0.6		0.6		0.6		(0.5)		(0.5)	18.1		10.3		18.1		10.3	7.9		2.1	7.9	2.1	66.4		68.1		66.4	68.1	0.1	0.1	0.1	0.1	0.1	0.1
The amount of total gains or (losses) for the period included in changes in net assets attributable to the change in unrealized gains or (losses) relating to assets and liabilities held at June 30	$ 0.7	$ 10.7	$ 0.5	$ 10.6					$ 3.8												$ (0.2)	[1]	$ 3.2	[1]	$ (0.4)	[1]	$ 1.6	[1]				$ 0.5	[1]	$ 13.9	[1]	$ 0.1		$ 12.2				$ 3.8

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