Fair Value Measurements (Fair Value Measurements Using Significant Unobservable Inputs) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018
Commodity Contracts Derivative Assets and (Liabilities), net [Member]
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance, January 1		$ 12.2	$ (12.2)
Total net gains (losses) included in changes in net assets (realized/unrealized)		8.2	9.1
Transfers out of Level 3	[1]	4.0	16.1
Purchases		13.8	26.7
Sales		(0.2)	(0.5)
Settlements		(16.8)	(27.0)
Ending balance, December 31		21.2	12.2
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 December 31		11.4	10.7
Commodity Contracts Derivative Assets and (Liabilities), net [Member] | IPL [Member]
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance, January 1		9.0	(1.4)
Total net gains (losses) included in changes in net assets (realized/unrealized)		10.7	(0.5)
Transfers out of Level 3	[1]	2.6	11.0
Purchases		9.5	22.5
Sales		(0.1)	(0.4)
Settlements		(13.0)	(22.2)
Ending balance, December 31		18.7	9.0
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 December 31		12.6	0.2
Commodity Contracts Derivative Assets and (Liabilities), net [Member] | WPL [Member]
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance, January 1		3.2	(10.8)
Total net gains (losses) included in changes in net assets (realized/unrealized)		(2.5)	9.6
Transfers out of Level 3	[1]	1.4	5.1
Purchases		4.3	4.2
Sales		(0.1)	(0.1)
Settlements		(3.8)	(4.8)
Ending balance, December 31		2.5	3.2
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 December 31		(1.2)	10.5
Deferred Proceeds [Member]
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance, January 1		119.4	222.1
Total net gains (losses) included in changes in net assets (realized/unrealized)		0.0	0.0
Transfers out of Level 3	[1]	0.0	0.0
Purchases		0.0	0.0
Sales		0.0	0.0
Settlements	[2]	68.3	(102.7)
Ending balance, December 31		187.7	119.4
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 December 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, January 1		119.4	222.1
Total net gains (losses) included in changes in net assets (realized/unrealized)		0.0	0.0
Transfers out of Level 3	[1]	0.0	0.0
Purchases		0.0	0.0
Sales		0.0	0.0
Settlements	[2]	68.3	(102.7)
Ending balance, December 31		187.7	119.4
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 December 31		$ 0.0	$ 0.0

[1] Observable market inputs became available for certain commodity contracts previously classified as Level 3 for transfers out of Level 3.

[2] 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.