Fair Value Measurements (Fair Value Measurements Using Significant Unobservable Inputs) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015
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		$ (32.7)	$ 17.9
Total net gains (losses) included in changes in net assets (realized/unrealized)		30.7	(63.5)
Transfers into Level 3		0.9	0.0
Transfers out of Level 3		1.2	0.3
Purchases		22.0	36.9
Sales		(1.0)	(1.9)
Settlements		(12.4)	(22.4)
Ending balance, December 31		8.7	(32.7)
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 December 31		32.7	(56.0)
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		(1.9)	19.4
Total net gains (losses) included in changes in net assets (realized/unrealized)		7.3	(29.6)
Transfers into Level 3		0.5	0.0
Transfers out of Level 3		0.2	0.0
Purchases		20.6	33.1
Sales		(1.0)	(1.8)
Settlements		(15.6)	(23.0)
Ending balance, December 31		10.1	(1.9)
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		8.5	(23.1)
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		(30.8)	(1.5)
Total net gains (losses) included in changes in net assets (realized/unrealized)		23.4	(33.9)
Transfers into Level 3		0.4	0.0
Transfers out of Level 3		1.0	0.3
Purchases		1.4	3.8
Sales		0.0	(0.1)
Settlements		3.2	0.6
Ending balance, December 31		(1.4)	(30.8)
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		24.2	(32.9)
Deferred Proceeds [Member]
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance, January 1		172.0	177.2
Total net gains (losses) included in changes in net assets (realized/unrealized)		0.0	0.0
Transfers into Level 3		0.0	0.0
Transfers out of Level 3		0.0	0.0
Purchases		0.0	0.0
Sales		0.0	0.0
Settlements	[1]	39.1	(5.2)
Ending balance, December 31		211.1	172.0
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		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		172.0	177.2
Total net gains (losses) included in changes in net assets (realized/unrealized)		0.0	0.0
Transfers into Level 3		0.0	0.0
Transfers out of Level 3		0.0	0.0
Purchases		0.0	0.0
Sales		0.0	0.0
Settlements	[1]	39.1	(5.2)
Ending balance, December 31		211.1	172.0
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		$ 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.