Fair Value Measurements (Fair Value Measurements Using Significant Unobservable Inputs) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011
Commodity Contracts Derivative Assets and (Liabilities), net [Member]
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance, January 1	$ (0.9)		$ 2.8
Total net losses (realized/unrealized) included in changes in net assets	(7.6)	[1]	(7.3)	[1]
Transfers into Level 3	(1.1)	[2]	0.2	[2]
Transfers out of Level 3	8.3	[3]	0	[3]
Purchases	35.8		21.8
Settlements	(22.6)	[4]	(18.4)	[4]
Ending balance, December 31	11.9		(0.9)
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 December 31	(2.6)	[1]	(7.3)	[1]
Commodity Contracts Derivative Assets and (Liabilities), net [Member] | IPL [Member]
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance, January 1	4.3		4.3
Total net losses (realized/unrealized) included in changes in net assets	(3.5)	[1]	(3.1)	[1]
Transfers into Level 3	(1.1)	[2]	0	[2]
Transfers out of Level 3	2.4	[3]	0	[3]
Purchases	26.8		18.1
Settlements	(16.4)	[4]	(15.0)	[4]
Ending balance, December 31	12.5		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 December 31	1.5	[1]	(3.1)	[1]
Commodity Contracts Derivative Assets and (Liabilities), net [Member] | WPL [Member]
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance, January 1	(5.2)		(1.5)
Total net losses (realized/unrealized) included in changes in net assets	(4.1)	[1]	(4.2)	[1]
Transfers into Level 3	0	[2]	0.2	[2]
Transfers out of Level 3	5.9	[3]	0	[3]
Purchases	9.0		3.7
Settlements	(6.2)		(3.4)
Ending balance, December 31	(0.6)		(5.2)
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 December 31	(4.1)	[1]	(4.2)	[1]
Deferred Proceeds [Member]
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance, January 1	53.7		152.9
Total net losses (realized/unrealized) included in changes in net assets	0	[1]	0	[1]
Transfers into Level 3	0	[2]	0	[2]
Transfers out of Level 3	0	[3]	0	[3]
Purchases	0		0
Settlements	13.1	[4]	(99.2)	[4]
Ending balance, December 31	66.8		53.7
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 December 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, January 1	53.7		152.9
Total net losses (realized/unrealized) included in changes in net assets	0	[1]	0	[1]
Transfers into Level 3	0	[2]	0	[2]
Transfers out of Level 3	0	[3]	0	[3]
Purchases	0		0
Settlements	13.1	[4]	(99.2)	[4]
Ending balance, December 31	66.8		53.7
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 December 31	$ 0	[1]	$ 0	[1]

[1] Gains and losses related to derivative assets and derivative liabilities are generally recorded in “Regulatory assets” and “Regulatory liabilities” on the 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.