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Summary of Significant Accounting Policies - MEC - Narrative (Details) - USD ($)
$ in Millions
Dec. 31, 2021
Feb. 28, 2021
Dec. 31, 2020
Allowance for Doubtful Accounts(1) [Line Items]      
Inventory, raw materials and supplies, gross $ 826   $ 786
Replacement cost of inventory, higher (lower) 27   10
Unbilled revenue 718   750
MEC      
Allowance for Doubtful Accounts(1) [Line Items]      
Inventory, raw materials and supplies, gross 135   129
Public utility inventory, coal 63   119
Public utility inventory - natural gas in storage 30   26
Replacement cost of inventory, higher (lower) 27   10
Unbilled revenue 85   95
Adjustment clause accounts receivable (payable) $ 230 $ 245 $ 22