Fair Value Measurements (Fair Value Measurements Using Significant Unobservable Inputs) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012
Commodity Contracts [Member]
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 42.5		$ 18.8		$ 11.9		$ (0.9)
Total net gains (losses) (realized/unrealized) included in changes in net assets	0.1	[1]	(2.3)	[1]	(8.3)	[1]	(8.3)	[1]
Transfers into Level 3	(9.9)	[2]	(0.4)	[2]	(0.4)	[2]	(1.7)	[2]
Transfers out of Level 3	0	[3]	9.3	[3]	(0.5)	[3]	8.3	[3]
Purchases					50.9		35.8
Settlements	(15.5)	[4]	(7.8)	[4]	(36.4)	[4]	(15.6)	[4]
Ending balance	17.2		17.6		17.2		17.6
The amount of total net 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 September 30	0.1	[1]	(0.7)	[1]	(8.3)	[1]	(4.4)	[1]
Commodity Contracts [Member] | IPL [Member]
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	40.6		14.1		12.5		4.3
Total net gains (losses) (realized/unrealized) included in changes in net assets	2.0	[1]	(0.2)	[1]	1.5	[1]	(4.8)	[1]
Transfers into Level 3	(0.1)	[2]	0	[2]	0	[2]	(1.1)	[2]
Transfers out of Level 3	0	[3]	7.4	[3]	(1.5)	[3]	2.4	[3]
Purchases					46.1		26.8
Settlements	(13.9)	[4]	(5.8)	[4]	(30.0)	[4]	(12.1)	[4]
Ending balance	28.6		15.5		28.6		15.5
The amount of total net 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 September 30	2.0	[1]	1.4	[1]	1.5	[1]	(0.7)	[1]
Commodity Contracts [Member] | WPL [Member]
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	1.9		4.7		(0.6)		(5.2)
Total net gains (losses) (realized/unrealized) included in changes in net assets	(1.9)	[1]	(2.1)	[1]	(9.8)	[1]	(3.5)	[1]
Transfers into Level 3	(9.8)	[2]	(0.4)	[2]	(0.4)	[2]	(0.6)	[2]
Transfers out of Level 3	0	[3]	1.9	[3]	1.0	[3]	5.9	[3]
Purchases					4.8		9.0
Settlements	(1.6)		(2.0)		(6.4)		(3.5)
Ending balance	(11.4)		2.1		(11.4)		2.1
The amount of total net 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 September 30	(1.9)	[1]	(2.1)	[1]	(9.8)	[1]	(3.7)	[1]
Deferred Proceeds [Member]
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	69.3		81.7		66.8		53.7
Total net gains (losses) (realized/unrealized) included in changes in net assets	0	[1]	0	[1]	0	[1]	0	[1]
Transfers into Level 3	0	[2]	0	[2]	0	[2]	0	[2]
Transfers out of Level 3	0	[3]	0	[3]	0	[3]	0	[3]
Purchases					0		0
Settlements	17.6	[4]	72.2	[4]	20.1	[4]	100.2	[4]
Ending balance	86.9		153.9		86.9		153.9
The amount of total net 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 September 30	0	[1]	0	[1]	0	[1]	0	[1]
Deferred Proceeds [Member] | IPL [Member]
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	69.3		81.7		66.8		53.7
Total net gains (losses) (realized/unrealized) included in changes in net assets	0	[1]	0	[1]	0	[1]	0	[1]
Transfers into Level 3	0	[2]	0	[2]	0	[2]	0	[2]
Transfers out of Level 3	0	[3]	0	[3]	0	[3]	0	[3]
Purchases					0		0
Settlements	17.6	[4]	72.2	[4]	20.1	[4]	100.2	[4]
Ending balance	86.9		153.9		86.9		153.9
The amount of total net 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 September 30	$ 0	[1]	$ 0	[1]	$ 0	[1]	$ 0	[1]

[1] Gains and 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 became inactive and observable market inputs became unavailable 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.