Regulatory Matters - Regulatory Assets (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2020		Dec. 31, 2019
Regulatory Assets [Line Items]
Regulatory Assets, Current		$ 283		$ 115
Regulatory assets, noncurrent		3,157		2,766
Total regulatory assets		3,440		2,881
Remaining Amounts of Regulatory Assets for which No Return on Investment During Recovery Period is Provided		1,600		1,400
Deferred Income Tax Charge [Member]
Regulatory Assets [Line Items]
Total regulatory assets		283	[1]	223
Employee benefit plans [Member]
Regulatory Assets [Line Items]
Total regulatory assets	[2]	$ 722		667
Regulatory asset amortization period years		15
Asset disposition costs [Member]
Regulatory Assets [Line Items]
Total regulatory assets		$ 347		391
Deferred net power costs [Member]
Regulatory Assets [Line Items]
Total regulatory assets		$ 139		110
Regulatory asset amortization period years		1 year
Asset Retirement Obligation Costs
Regulatory Assets [Line Items]
Total regulatory assets		$ 640		445
Regulatory asset amortization period years		13 years
Deferred Operating Costs
Regulatory Assets [Line Items]
Total regulatory assets		$ 124		134
Regulatory asset amortization period years		11 years
Regulatory assets other [Member]
Regulatory Assets [Line Items]
Total regulatory assets		$ 988		902
us-gaap_demandsidemanagement
Regulatory Assets [Line Items]
Total regulatory assets		$ 197		$ 9
Regulatory asset amortization period years		10 years

[1] Amounts primarily represent income tax benefits related to certain property-related basis differences and other various differences that were previously passed on to customers and will be included in regulated rates when the temporary differences reverse.

[2] Represents amounts not yet recognized as a component of net periodic benefit cost that are expected to be included in regulated rates when recognized.