Segment Information - MEC (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($) reportableSegment operatingSegment	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments | operatingSegment			8
Operating revenue	$ 6,968	$ 6,193	$ 19,113	$ 15,332
Operating income	1,834	1,547	4,375	3,402
Interest expense	(531)	(504)	(1,593)	(1,490)
Allowance for equity funds	34	50	90	122
Other, net	8	36	64	61
Income (loss) before income tax expense (benefit) and equity income (loss)	1,675	2,967	4,189	4,619
Assets	131,554		$ 131,554		$ 127,316
MEC
Segment Reporting Information [Line Items]
Number of reportable segments | reportableSegment			2
Operating revenue	966	812	$ 2,726	2,106
Operating income	287	232	438	438
Interest expense	(76)	(74)	(224)	(224)
Allowance for borrowed funds	4	5	8	12
Allowance for equity funds	11	16	25	33
Other, net	8	14	34	30
Income (loss) before income tax expense (benefit) and equity income (loss)	234	193	281	289
Assets	22,944		22,944		21,437
MEC | Regulated electric
Segment Reporting Information [Line Items]
Operating revenue	854	728	1,985	1,717
Operating income	289	238	401	398
Assets	21,063		21,063		19,892
MEC | Regulated natural gas
Segment Reporting Information [Line Items]
Operating revenue	110	80	728	384
Operating income	(2)	(6)	37	40
Assets	1,874		1,874		1,544
MEC | Other
Segment Reporting Information [Line Items]
Operating revenue	2	$ 4	13	$ 5
Assets	$ 7		$ 7		$ 1