Contingencies (Detail) $ in Millions						1 Months Ended		9 Months Ended
	May 21, 2020	May 20, 2020	Mar. 22, 2016	Mar. 03, 2015	Oct. 16, 2014	Oct. 31, 2018 complaint	Apr. 30, 2018	Sep. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)	May 01, 2018 USD ($)
Loss Contingencies [Line Items]
Regulatory liabilities								$ 3,342	$ 3,281
Requested existing base return on equity base percentage	10.02%	9.88%
Price of the power purchase agreements								259
Requested renewables delay from preliminary proposed ruling period							2 years
Standby letters of credit								449
Amphora Gas Storage USA, LLC
Loss Contingencies [Line Items]
Claim for indemnification										$ 20
Indemnification liability, percentage of purchase price										15.00%
Indemnification liability, maximum amount										$ 10
Complaint III
Loss Contingencies [Line Items]
Regulatory liabilities								7
Complaint II
Loss Contingencies [Line Items]
Regulatory liabilities								26
Complaint II and III
Loss Contingencies [Line Items]
Reasonably possible loss, in additional reserve, pre tax								$ 17
Unfavorable Regulatory Action | Complaint I
Loss Contingencies [Line Items]
Approved return on equity			9.59%		10.57%	9.88%
Refund period term								15 months
Number of claims | complaint						4
Unfavorable Regulatory Action | Complaint I | Max.
Loss Contingencies [Line Items]
Approved return on equity			10.42%	11.74%	11.74%
Unfavorable Regulatory Action | Complaint III
Loss Contingencies [Line Items]
Approved return on equity			10.90%
Unfavorable Regulatory Action | Complaint III | Max.
Loss Contingencies [Line Items]
Approved return on equity			12.19%
Before Amendment
Loss Contingencies [Line Items]
Approved return on equity								11.14%