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Fair Value Measurements - Level 3 (Details) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Commodity derivative [Member]    
Fair Value, Assets (Liabilities), Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Beginning balance $ 116 $ 97
Changes included in earnings (6) (3)
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Gain (Loss) Included in Other Comprehensive Income (Loss) 1  
Changes in fair value recognized in net regulatory assets 16 (40)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases   2
Settlements (1) (4)
Ending balance 124 52
Interest Rate Lock Commitments [Member]    
Fair Value, Assets (Liabilities), Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Beginning balance 62 14
Changes included in earnings (21) 31
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Gain (Loss) Included in Other Comprehensive Income (Loss) 0  
Changes in fair value recognized in net regulatory assets 0 0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases   0
Settlements 0 0
Ending balance $ 41 $ 45