Fair Value Measurements (Fair Value Measurements Using Significant Unobservable Inputs) (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Commodity Contracts [Member]
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance		$ (65.9)	$ (13.1)	$ (32.7)	$ 17.9
Total net gains (losses) included in changes in net assets (realized/unrealized)		44.6	(19.6)	13.1	(36.8)
Transfers into Level 3				0.9	0.0
Transfers out of Level 3		0.4	1.0	0.4	0.6
Purchases		22.0	36.8	22.0	36.8
Sales		(0.1)	(0.4)	(0.7)	(1.2)
Settlements	[1]	(0.4)	(4.1)	(2.4)	(16.7)
Ending balance		0.6	0.6	0.6	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 June 30		44.8	(17.7)	14.8	(33.9)
Commodity Contracts [Member] | IPL [Member]
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance		(13.1)	(5.0)	(1.9)	19.4
Total net gains (losses) included in changes in net assets (realized/unrealized)		12.9	(5.0)	5.2	(17.4)
Transfers into Level 3				0.5	0.0
Transfers out of Level 3		(0.1)	0.2	(0.1)	0.0
Purchases		20.6	33.1	20.6	33.1
Sales		(0.1)	(0.4)	(0.7)	(1.1)
Settlements	[1]	(1.9)	(4.6)	(5.3)	(15.7)
Ending balance		18.3	18.3	18.3	18.3
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		12.8	(4.2)	6.2	(14.3)
Commodity Contracts [Member] | WPL [Member]
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance		(52.8)	(8.1)	(30.8)	(1.5)
Total net gains (losses) included in changes in net assets (realized/unrealized)		31.7	(14.6)	7.9	(19.4)
Transfers into Level 3				0.4	0.0
Transfers out of Level 3		0.5	0.8	0.5	0.6
Purchases		1.4	3.7	1.4	3.7
Sales				0.0	(0.1)
Settlements		1.5	0.5	2.9	(1.0)
Ending balance		(17.7)	(17.7)	(17.7)	(17.7)
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 June 30		32.0	(13.5)	8.6	(19.6)
Deferred Proceeds [Member]
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance		154.2	117.7	172.0	177.2
Total net gains (losses) included in changes in net assets (realized/unrealized)		0.0	0.0	0.0	0.0
Transfers into Level 3				0.0	0.0
Transfers out of Level 3		0.0	0.0	0.0	0.0
Purchases		0.0	0.0	0.0	0.0
Sales		0.0	0.0	0.0	0.0
Settlements	[1]	(79.8)	(44.3)	(97.6)	(103.8)
Ending balance		74.4	73.4	74.4	73.4
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 June 30		0.0	0.0	0.0	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		154.2	117.7	172.0	177.2
Total net gains (losses) included in changes in net assets (realized/unrealized)		0.0	0.0	0.0	0.0
Transfers into Level 3				0.0	0.0
Transfers out of Level 3		0.0	0.0	0.0	0.0
Purchases		0.0	0.0	0.0	0.0
Sales		0.0	0.0	0.0	0.0
Settlements	[1]	(79.8)	(44.3)	(97.6)	(103.8)
Ending balance		74.4	73.4	74.4	73.4
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 June 30		$ 0.0	$ 0.0	$ 0.0	$ 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.