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Fair Value Measurements (Fair Value Measurements Using Significant Unobservable Inputs) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014
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		$ 17.9	$ 4.4
Total net gains (losses) included in changes in net assets (realized/unrealized)		(63.5)	11.1
Transfers out of Level 3		0.3	0.0
Purchases		36.9	76.7
Sales		(1.9)	(2.2)
Settlements		(22.4)	(72.1)
Ending balance, December 31		(32.7)	17.9
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		(56.0)	(0.4)
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		19.4	14.6
Total net gains (losses) included in changes in net assets (realized/unrealized)		(29.6)	(5.9)
Purchases		33.1	68.8
Sales		(1.8)	(2.0)
Settlements		(23.0)	(56.1)
Ending balance, December 31		(1.9)	19.4
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		(23.1)	(9.3)
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		(1.5)	(10.2)
Total net gains (losses) included in changes in net assets (realized/unrealized)		(33.9)	17.0
Transfers out of Level 3		0.3	0.0
Purchases		3.8	7.9
Sales		(0.1)	(0.2)
Settlements		0.6	(16.0)
Ending balance, December 31		(30.8)	(1.5)
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		(32.9)	8.9
Deferred Proceeds [Member]
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance, January 1		177.2	203.5
Total net gains (losses) included in changes in net assets (realized/unrealized)		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]	(5.2)	(26.3)
Ending balance, December 31		172.0	177.2
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		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		177.2	203.5
Total net gains (losses) included in changes in net assets (realized/unrealized)		0.0	0.0
Purchases		0.0	0.0
Sales		0.0	0.0
Settlements	[1]	(5.2)	(26.3)
Ending balance, December 31		172.0	177.2
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		$ 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.

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Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]

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The portion (amount) of unrealized gains and losses recognized as changes in net assets during the period and attributable to alternative investments continued to be held at period end. These gains or losses are attributable to certain investments for which net asset value per share is calculated for purposes of measuring fair value and which are subject to unobservable inputs.

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-Publisher FASB
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Amount of gain (loss) recognized in the income statement for financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of purchases of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of settlement of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of transfers of financial instrument classified as an asset out of level 3 of the fair value hierarchy.

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