Fair Value Measurements (Fair Value Measurements Using Significant Unobservable Inputs) (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Commodity Contracts [Member]
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance		$ (29.4)	$ (32.9)	$ (12.2)	$ 8.7
Total net gains (losses) included in changes in net assets (realized/unrealized)		(0.2)	8.1	(10.0)	(27.0)
Transfers out of Level 3		0.0	12.2	0.0	12.2
Purchases		26.7	28.3	26.7	28.3
Sales				0.0	(0.2)
Settlements		(7.8)	(6.5)	(15.2)	(12.8)
Ending balance		(10.7)	9.2	(10.7)	9.2
The amount of total net gains (losses) for the period included in changes in net assets attributable to the change in unrealized losses relating to assets and liabilities held at June 30		(0.1)	8.3	(9.7)	(25.4)
Commodity Contracts [Member] | IPL [Member]
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance		(15.4)	(8.3)	(1.4)	10.1
Total net gains (losses) included in changes in net assets (realized/unrealized)		(1.6)	2.9	(9.2)	(9.5)
Transfers out of Level 3		0.0	3.4	0.0	3.1
Purchases		19.3	24.6	19.3	24.6
Sales				0.0	(0.1)
Settlements		(6.4)	(5.5)	(12.8)	(11.1)
Ending balance		(4.1)	17.1	(4.1)	17.1
The amount of total net gains (losses) for the period included in changes in net assets attributable to the change in unrealized losses relating to assets and liabilities held at June 30		(1.6)	2.9	(9.0)	(8.2)
Commodity Contracts [Member] | WPL [Member]
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance		(14.0)	(24.6)	(10.8)	(1.4)
Total net gains (losses) included in changes in net assets (realized/unrealized)		1.4	5.2	(0.8)	(17.5)
Transfers out of Level 3		0.0	8.8	0.0	9.1
Purchases		7.4	3.7	7.4	3.7
Sales				0.0	(0.1)
Settlements		(1.4)	(1.0)	(2.4)	(1.7)
Ending balance		(6.6)	(7.9)	(6.6)	(7.9)
The amount of total net gains (losses) for the period included in changes in net assets attributable to the change in unrealized losses relating to assets and liabilities held at June 30		1.5	5.4	(0.7)	(17.2)
Deferred Proceeds [Member]
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance		120.9	149.0	222.1	211.1
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	0.0	0.0
Sales				0.0	0.0
Settlements	[1]	87.4	21.0	(13.8)	(41.1)
Ending balance		208.3	170.0	208.3	170.0
The amount of total net gains (losses) for the period included in changes in net assets attributable to the change in unrealized 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		120.9	149.0	222.1	211.1
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	0.0	0.0
Sales				0.0	0.0
Settlements	[1]	87.4	21.0	(13.8)	(41.1)
Ending balance		208.3	170.0	208.3	170.0
The amount of total net gains (losses) for the period included in changes in net assets attributable to the change in unrealized 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 amounts received from the receivables sold.