Revenue Recognition and Allowance for Credit Losses - Summary of Contract Assets and Liabilities (Details) $ in Millions	6 Months Ended
Jun. 30, 2021 USD ($)
Change in Accounts Receivable [Roll Forward]
Opening balance as of December 31, 2020	$ 604
Closing balance as of June 30, 2021	505
Increase (Decrease) in Accounts Receivable on Contracts with Customers	(99)
Change in Other Accrued Unbilled Revenues [Roll Forward]
Opening balance as of December 31, 2020	505
Closing balance as of June 30, 2021	272
Increase (Decrease) in Unbilled Receivables	(233)
Change in Contract Assets [Roll Forward]
Opening balance as of December 31, 2020	27
Closing balance as of June 30, 2021	24
Increase (Decrease) in Contract with Customer, Asset	(3)
Change in Contract Liability [Roll Forward]
Opening balance as of December 31, 2020	18
Closing balance as of June 30, 2021	22
Increase (Decrease) in Contract with Customer, Liability	4
Revenue recognized included in the opening contract liability for the period	15
Houston Electric
Change in Accounts Receivable [Roll Forward]
Opening balance as of December 31, 2020	225
Closing balance as of June 30, 2021	260
Increase (Decrease) in Accounts Receivable on Contracts with Customers	35
Change in Other Accrued Unbilled Revenues [Roll Forward]
Opening balance as of December 31, 2020	113
Closing balance as of June 30, 2021	116
Increase (Decrease) in Unbilled Receivables	3
Change in Contract Liability [Roll Forward]
Opening balance as of December 31, 2020	3
Closing balance as of June 30, 2021	6
Increase (Decrease) in Contract with Customer, Liability	3
Revenue recognized included in the opening contract liability for the period	2
CERC Corp
Change in Accounts Receivable [Roll Forward]
Opening balance as of December 31, 2020	214
Closing balance as of June 30, 2021	114
Increase (Decrease) in Accounts Receivable on Contracts with Customers	(100)
Change in Other Accrued Unbilled Revenues [Roll Forward]
Opening balance as of December 31, 2020	261
Closing balance as of June 30, 2021	81
Increase (Decrease) in Unbilled Receivables	$ (180)

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Amount, after allowance for credit loss, of right to consideration in exchange for good or service transferred to customer when right is conditioned on something other than passage of time, classified as current.

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Unbilled amounts due for services rendered or to be rendered, actions taken or to be taken, or a promise to refrain from taking certain actions in accordance with the terms of a legally binding agreement between the entity and, at a minimum, one other party. An example would be amounts associated with contracts or programs where the recognized revenue for performance thereunder exceeds the amounts billed under the terms thereof as of the date of the balance sheet.

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