Fair Value Measurements (Fair Value Measurements Using Significant Unobservable Inputs) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2014		Jun. 30, 2013		Jun. 30, 2014		Jun. 30, 2013
Commodity Contracts [Member]
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 23.9		$ 4.9		$ 4.4		$ 11.9
Total net gains (losses) (realized/unrealized) included in changes in net assets	16.9	[1]	(1.2)	[1]	54.7	[1]	(3.0)	[1]
Transfers out of Level 3					0	[2]	3.6	[2]
Purchases	76.7		50.9		(76.7)		50.9
Settlements	(16.3)	[3]	(12.1)	[3]	(34.6)	[3]	(20.9)	[3]
Ending balance	101.2		42.5		101.2		42.5
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 June 30	16.3	[1]	(1.2)	[1]	46.0	[1]	(3.0)	[1]
Commodity Contracts [Member] | IPL [Member]
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	7.8		4.3		14.6		12.5
Total net gains (losses) (realized/unrealized) included in changes in net assets	0.6	[1]	(0.1)	[1]	4.4	[1]	(3.0)	[1]
Transfers out of Level 3					0	[2]	1.1	[2]
Purchases	68.8		46.1		68.8		46.1
Settlements	(13.0)	[3]	(9.7)	[3]	(23.6)	[3]	(16.1)	[3]
Ending balance	64.2		40.6		64.2		40.6
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 June 30	0.7	[1]	(0.1)	[1]	3.7	[1]	(3.0)	[1]
Commodity Contracts [Member] | WPL [Member]
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	16.1		0.6		(10.2)		(0.6)
Total net gains (losses) (realized/unrealized) included in changes in net assets	16.3	[1]	(1.1)	[1]	50.3	[1]	0	[1]
Transfers out of Level 3					0	[2]	2.5	[2]
Purchases	7.9		4.8		7.9		4.8
Settlements	(3.3)		(2.4)		(11.0)		(4.8)
Ending balance	37.0		1.9		37.0		1.9
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 June 30	15.6	[1]	(1.1)	[1]	42.3	[1]	0	[1]
Deferred Proceeds [Member]
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	170.8		122.1		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 out of Level 3					0	[2]	0	[2]
Purchases	0		0		0		0
Settlements	22.9	[3]	(52.8)	[3]	(9.8)	[3]	2.5	[3]
Ending balance	193.7		69.3		193.7		69.3
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 June 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	170.8		122.1		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 out of Level 3					0	[2]	0	[2]
Purchases	0		0		0		0
Settlements	22.9	[3]	(52.8)	[3]	(9.8)	[3]	2.5	[3]
Ending balance	193.7		69.3		193.7		69.3
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 June 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 balance sheets.

[2] 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.

[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.