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Fair Value of Financial Instruments - Reconciliation of Level 3 Financial Instruments (Details) - Level 3 - Derivatives - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2022
Mar. 31, 2021
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Beginning balance $ 293 $ (16)
Contracts added from Direct Energy acquisition 0 (15)
Total gains realized/unrealized — included in earnings 166 180
Purchases 23 20
Transfers into Level 3 53 4
Transfers out of Level 3 (7) (14)
Ending balance 528 159
Gains for the period included in earnings attributable to the change in unrealized gains or losses relating to assets or liabilities still held as of period end $ 237 $ 146