Fair Value Measurements (Fair Value Measurements Using Significant Unobservable Inputs) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015		Mar. 31, 2014
Commodity Contracts [Member]
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 17.9		$ 4.4
Total net gains (losses) (realized/unrealized) included in changes in net assets	(18.1)		38.4
Sales	(0.9)		0
Settlements	(12.0)	[1]	(18.9)	[1]
Ending balance	(13.1)		23.9
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	(16.5)		31.0
Commodity Contracts [Member] | IPL [Member]
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	19.4		14.6
Total net gains (losses) (realized/unrealized) included in changes in net assets	(12.5)		4.2
Sales	(0.9)		0
Settlements	(11.0)	[1]	(11.0)	[1]
Ending balance	(5.0)		7.8
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	(10.7)		3.1
Commodity Contracts [Member] | WPL [Member]
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	(1.5)		(10.2)
Total net gains (losses) (realized/unrealized) included in changes in net assets	(5.6)		34.2
Settlements	(1.0)		(7.9)
Ending balance	(8.1)		16.1
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	(5.8)		27.9
Deferred Proceeds [Member]
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	177.2		203.5
Total net gains (losses) (realized/unrealized) included in changes in net assets	0		0
Sales	0		0
Settlements	(59.5)	[1]	(32.7)	[1]
Ending balance	117.7		170.8
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		0
Deferred Proceeds [Member] | IPL [Member]
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	177.2		203.5
Total net gains (losses) (realized/unrealized) included in changes in net assets	0		0
Sales	0		0
Settlements	(59.5)	[1]	(32.7)	[1]
Ending balance	117.7		170.8
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		$ 0

[1] 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.