Fair Value Measurements (Fair Value Measurements Using Significant Unobservable Inputs) (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2019	Mar. 31, 2018
Commodity Contracts [Member]
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance		$ 12.2	$ (12.2)
Total net losses included in changes in net assets (realized/unrealized)		(5.6)	(9.8)
Sales		(0.2)	0.0
Settlements		(6.0)	(7.4)
Ending balance		0.4	(29.4)
The amount of total net losses for the period included in changes in net assets attributable to the change in unrealized losses relating to assets and liabilities held at March 31		(2.6)	(9.4)
Commodity Contracts [Member] | IPL [Member]
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance		9.0	(1.4)
Total net losses included in changes in net assets (realized/unrealized)		(3.2)	(7.6)
Sales		(0.1)	0.0
Settlements		(5.1)	(6.4)
Ending balance		0.6	(15.4)
The amount of total net losses for the period included in changes in net assets attributable to the change in unrealized losses relating to assets and liabilities held at March 31		(1.4)	(7.3)
Commodity Contracts [Member] | WPL [Member]
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance		3.2	(10.8)
Total net losses included in changes in net assets (realized/unrealized)		(2.4)	(2.2)
Sales		(0.1)	0.0
Settlements		(0.9)	(1.0)
Ending balance		(0.2)	(14.0)
The amount of total net losses for the period included in changes in net assets attributable to the change in unrealized losses relating to assets and liabilities held at March 31		(1.2)	(2.1)
Deferred Proceeds [Member]
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance		119.4	222.1
Total net losses included in changes in net assets (realized/unrealized)		0.0	0.0
Sales		0.0	0.0
Settlements	[1]	58.9	(101.2)
Ending balance		178.3	120.9
The amount of total net losses for the period included in changes in net assets attributable to the change in unrealized losses relating to assets and liabilities held at March 31		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		119.4	222.1
Total net losses included in changes in net assets (realized/unrealized)		0.0	0.0
Sales		0.0	0.0
Settlements	[1]	58.9	(101.2)
Ending balance		178.3	120.9
The amount of total net losses for the period included in changes in net assets attributable to the change in unrealized losses relating to assets and liabilities held at March 31		$ 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.