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, 2014		Sep. 30, 2013		Sep. 30, 2014		Sep. 30, 2013
Commodity Contracts [Member]
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 101.2		$ 42.5		$ 4.4		$ 11.9
Total net gains (losses) (realized/unrealized) included in changes in net assets	(12.7)	[1]	0.1	[1]	43.0	[1]	(8.3)	[1]
Transfers into Level 3	0	[2]	(9.9)	[2]	0	[2]	(0.4)	[2]
Transfers out of Level 3					0	[3]	(0.5)	[3]
Purchases					76.7		50.9
Sales	(1.2)		0		(1.2)		0
Settlements	(19.0)	[4]	(15.5)	[4]	(54.6)	[4]	(36.4)	[4]
Ending balance	68.3		17.2		68.3		17.2
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	(10.3)	[1]	0.1	[1]	34.6	[1]	(8.3)	[1]
Commodity Contracts [Member] | IPL [Member]
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	64.2		40.6		14.6		12.5
Total net gains (losses) (realized/unrealized) included in changes in net assets	(10.1)	[1]	2.0	[1]	(5.1)	[1]	1.5	[1]
Transfers into Level 3	0	[2]	(0.1)	[2]
Transfers out of Level 3					0	[3]	(1.5)	[3]
Purchases					68.8		46.1
Sales	(1.0)		0		(1.0)		0
Settlements	(16.6)	[4]	(13.9)	[4]	(40.8)	[4]	(30.0)	[4]
Ending balance	36.5		28.6		36.5		28.6
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	(9.6)	[1]	2.0	[1]	(6.0)	[1]	1.5	[1]
Commodity Contracts [Member] | WPL [Member]
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	37.0		1.9		(10.2)		(0.6)
Total net gains (losses) (realized/unrealized) included in changes in net assets	(2.6)	[1]	(1.9)	[1]	48.1	[1]	(9.8)	[1]
Transfers into Level 3	0	[2]	(9.8)	[2]	0	[2]	(0.4)	[2]
Transfers out of Level 3					0	[3]	1.0	[3]
Purchases					7.9		4.8
Sales	(0.2)		0		(0.2)		0
Settlements	(2.4)		(1.6)		(13.8)		(6.4)
Ending balance	31.8		(11.4)		31.8		(11.4)
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	(0.7)	[1]	(1.9)	[1]	40.6	[1]	(9.8)	[1]
Deferred Proceeds [Member]
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	193.7		69.3		203.5		66.8
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]
Purchases					0		0
Sales	0		0		0		0
Settlements	(33.4)	[4]	17.6	[4]	(43.2)	[4]	20.1	[4]
Ending balance	160.3		86.9		160.3		86.9
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	193.7		69.3		203.5		66.8
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]
Transfers out of Level 3					0	[3]	0	[3]
Purchases					0		0
Sales	0		0		0		0
Settlements	(33.4)	[4]	17.6	[4]	(43.2)	[4]	20.1	[4]
Ending balance	160.3		86.9		160.3		86.9
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] Unrealized gains and losses related to derivative assets and derivative liabilities are recorded in regulatory assets and regulatory liabilities on the 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.