Regulatory Matters - Regulatory Assets (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018
Regulatory Assets [Line Items]
Regulatory assets		$ 115	$ 171
Regulatory assets, noncurrent		2,766	2,896
Total regulatory assets		2,881	3,067
Remaining Amounts of Regulatory Assets for which No Return on Investment During Recovery Period is Provided		1,400	1,300
Deferred Income Tax Charge [Member]
Regulatory Assets [Line Items]
Total regulatory assets	[1]	$ 223	196
Regulatory asset amortization period years	[1]	Various
Employee benefit plans [Member]
Regulatory Assets [Line Items]
Total regulatory assets	[2]	$ 667	773
Regulatory asset amortization period years	[2]	15.2053973013493
Asset disposition costs [Member]
Regulatory Assets [Line Items]
Total regulatory assets		$ 391	358
Regulatory asset amortization period years		Various
Deferred net power costs [Member]
Regulatory Assets [Line Items]
Total regulatory assets		$ 110	103
Regulatory asset amortization period years		1.92727272727273
Asset retirement obligations
Regulatory Assets [Line Items]
Total regulatory assets		$ 445	375
Regulatory asset amortization period years		Various
Unrealized loss on regulated derivative contracts [Member]
Regulatory Assets [Line Items]
Total regulatory assets		$ 78	120
Regulatory asset amortization period years		2.58974358974359
Abandoned projects
Regulatory Assets [Line Items]
Total regulatory assets		$ 58	134
Regulatory asset amortization period years		3.48275862068966
Deferred Operating Costs
Regulatory Assets [Line Items]
Total regulatory assets		$ 134	141
Regulatory asset amortization period years		11
Unamortized contract values [Member]
Regulatory Assets [Line Items]
Total regulatory assets		$ 60	79
Regulatory asset amortization period years		4
Regulatory assets other [Member]
Regulatory Assets [Line Items]
Total regulatory assets		$ 715	$ 788
Regulatory asset amortization period years		Various

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