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Fair Value Measurements (Fair Value Measurements Using Significant Unobservable Inputs) (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Commodity Contracts [Member]    
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Beginning balance $ 8.7 $ (32.7)
Total net losses included in changes in net assets (realized/unrealized) (35.2) (31.4)
Transfers into Level 3 0.0 0.9
Sales (0.1) (0.6)
Settlements (6.3) (2.1)
Ending balance (32.9) (65.9)
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 (33.7) (30.0)
Commodity Contracts [Member] | IPL [Member]    
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Beginning balance 10.1 (1.9)
Total net losses included in changes in net assets (realized/unrealized) (12.7) (7.6)
Transfers into Level 3 0.0 0.5
Sales (0.1) (0.6)
Settlements (5.6) (3.5)
Ending balance (8.3) (13.1)
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 (11.4) (6.6)
Commodity Contracts [Member] | WPL [Member]    
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Beginning balance (1.4) (30.8)
Total net losses included in changes in net assets (realized/unrealized) (22.5) (23.8)
Transfers into Level 3 0.0 0.4
Settlements (0.7) 1.4
Ending balance (24.6) (52.8)
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 (22.3) (23.4)
Deferred Proceeds [Member]    
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Beginning balance 211.1 172.0
Total net losses included in changes in net assets (realized/unrealized) 0.0 0.0
Transfers into Level 3 0.0 0.0
Sales 0.0 0.0
Settlements [1] (62.1) (17.8)
Ending balance 149.0 154.2
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 211.1 172.0
Total net losses included in changes in net assets (realized/unrealized) 0.0 0.0
Transfers into Level 3 0.0 0.0
Sales 0.0 0.0
Settlements [1] (62.1) (17.8)
Ending balance 149.0 154.2
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.