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Fair Value Measurements (Fair Value Measurements Using Significant Unobservable Inputs) (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Commodity Contracts [Member]    
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Beginning balance $ 29 $ 21
Total net losses included in changes in net assets (realized/unrealized) (6) (3)
Settlements (7) (4)
Ending balance 16 14
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 (6) (3)
Commodity Contracts [Member] | IPL [Member]    
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Beginning balance 26 18
Total net losses included in changes in net assets (realized/unrealized) (5) (2)
Settlements (6) (3)
Ending balance 15 13
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 (6) (2)
Deferred Proceeds [Member]    
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Beginning balance 188 188
Total net losses included in changes in net assets (realized/unrealized) 0 0
Settlements (81) 0
Ending balance 107 188
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
Deferred Proceeds [Member] | IPL [Member]    
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Beginning balance 188 188
Total net losses included in changes in net assets (realized/unrealized) 0 0
Settlements (81) 0
Ending balance 107 188
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