Fair Value Measurements (Fair Value Measurements Using Significant Unobservable Inputs) (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Commodity Contracts [Member]
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance		$ 13.2	$ (10.7)	$ 12.2	$ (12.2)
Total net gains (losses) included in changes in net assets (realized/unrealized)		12.7	25.7	6.9	15.7
Transfers out of Level 3		(1.2)	15.6	2.8	15.6
Purchases				13.8	26.7
Sales		0.0	(0.2)	(0.2)	(0.2)
Settlements		(3.0)	(6.2)	(13.8)	(21.4)
Ending balance		21.7	24.2	21.7	24.2
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 September 30		12.7	26.1	10.1	16.5
Commodity Contracts [Member] | IPL [Member]
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance		9.3	(4.1)	9.0	(1.4)
Total net gains (losses) included in changes in net assets (realized/unrealized)		11.5	16.8	8.6	7.6
Transfers out of Level 3		(0.6)	10.5	2.0	10.5
Purchases				9.5	19.3
Sales		0.0	(0.1)	(0.1)	(0.1)
Settlements		(2.2)	(4.9)	(11.0)	(17.7)
Ending balance		18.0	18.2	18.0	18.2
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 September 30		11.5	16.8	10.5	7.9
Commodity Contracts [Member] | WPL [Member]
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance		3.9	(6.6)	3.2	(10.8)
Total net gains (losses) included in changes in net assets (realized/unrealized)		1.2	8.9	(1.7)	8.1
Transfers out of Level 3		(0.6)	5.1	0.8	5.1
Purchases				4.3	7.4
Sales		0.0	(0.1)	(0.1)	(0.1)
Settlements		(0.8)	(1.3)	(2.8)	(3.7)
Ending balance		3.7	6.0	3.7	6.0
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 September 30		1.2	9.3	(0.4)	8.6
Deferred Proceeds [Member]
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance		214.6	208.3	119.4	222.1
Total net gains (losses) included in changes in net assets (realized/unrealized)		0.0	0.0	0.0	0.0
Transfers out of Level 3		0.0	0.0	0.0	0.0
Purchases				0.0	0.0
Sales		0.0	0.0	0.0	0.0
Settlements	[1]	22.4	35.4	117.6	21.6
Ending balance		237.0	243.7	237.0	243.7
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 September 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		214.6	208.3	119.4	222.1
Total net gains (losses) included in changes in net assets (realized/unrealized)		0.0	0.0	0.0	0.0
Transfers out of Level 3		0.0	0.0	0.0	0.0
Purchases				0.0	0.0
Sales		0.0	0.0	0.0	0.0
Settlements	[1]	22.4	35.4	117.6	21.6
Ending balance		237.0	243.7	237.0	243.7
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 September 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 amounts received from the receivables sold.