Fair Value Measurements (Fair Value Measurements Using Significant Unobservable Inputs) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014		Mar. 31, 2013
Commodity Contracts [Member]
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 4.4		$ 11.9
Total net gains (losses) (realized/unrealized) included in changes in net assets	38.4	[1]	(2.4)	[1]
Transfers out of Level 3	0	[2]	3.6	[2]
Settlements	(18.9)	[3]	(8.2)	[3]
Ending balance	23.9		4.9
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 March 31	31.0	[1]	(2.4)	[1]
Commodity Contracts [Member] | IPL [Member]
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	14.6		12.5
Total net gains (losses) (realized/unrealized) included in changes in net assets	4.2	[1]	(2.9)	[1]
Transfers out of Level 3	0	[2]	1.1	[2]
Settlements	(11.0)	[3]	(6.4)	[3]
Ending balance	7.8		4.3
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 March 31	3.1	[1]	(2.9)	[1]
Commodity Contracts [Member] | WPL [Member]
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	(10.2)		(0.6)
Total net gains (losses) (realized/unrealized) included in changes in net assets	34.2	[1]	0.5	[1]
Transfers out of Level 3	0	[2]	2.5	[2]
Settlements	(7.9)		(1.8)
Ending balance	16.1		0.6
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 March 31	27.9	[1]	0.5	[1]
Deferred Proceeds [Member]
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	203.5		66.8
Total net gains (losses) (realized/unrealized) included in changes in net assets	0	[1]	0	[1]
Transfers out of Level 3	0	[2]	0	[2]
Settlements	(32.7)	[3]	55.3	[3]
Ending balance	170.8		122.1
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 March 31	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	203.5		66.8
Total net gains (losses) (realized/unrealized) included in changes in net assets	0	[1]	0	[1]
Transfers out of Level 3	0	[2]	0	[2]
Settlements	(32.7)	[3]	55.3	[3]
Ending balance	170.8		122.1
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 March 31	$ 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.