Segment Information - MEC (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2021 USD ($) operatingSegment reportableSegment	Jun. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments | operatingSegment			8
Operating revenue	$ 6,064	$ 4,612	$ 12,145	$ 9,139
Operating income	1,208	936	2,541	1,855
Interest expense	(532)	(503)	(1,062)	(986)
Allowance for equity funds	30	38	56	72
Other, net	48	52	56	25
Income (loss) before income tax expense (benefit) and equity income (loss)	2,760	1,145	2,514	1,652
Assets	130,561		$ 130,561		$ 127,316
MEC
Segment Reporting Information [Line Items]
Number of reportable segments | reportableSegment			2
Operating revenue	693	613	$ 1,760	1,294
Operating income	103	108	151	206
Interest expense	(74)	(74)	(148)	(150)
Allowance for borrowed funds	2	4	4	7
Allowance for equity funds	8	9	14	17
Other, net	15	21	26	16
Income (loss) before income tax expense (benefit) and equity income (loss)	54	68	47	96
Assets	22,057		22,057		21,437
MEC | Electric
Segment Reporting Information [Line Items]
Operating revenue	586	518	1,131	989
Operating income	103	101	112	160
Assets	20,349		20,349		19,892
MEC | Natural Gas
Segment Reporting Information [Line Items]
Operating revenue	106	95	618	304
Operating income	0	7	39	46
Assets	1,708		1,708		1,544
MEC | Other
Segment Reporting Information [Line Items]
Operating revenue	1	0	11	1
Operating income	0	$ 0	0	$ 0
Assets	$ 0		$ 0		$ 1