Short-term Debt and Credit Facilities - MEC (Details) $ in Millions	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)
Line of Credit Facility [Line Items]
Credit facilities	$ 11,281	$ 11,030
Tax-exempt bond support and letters of credit	(589)	(590)
Net credit facilities	8,683	8,154
MEC
Line of Credit Facility [Line Items]
Credit facilities	1,505	1,505
Tax-exempt bond support and letters of credit	(370)	(370)
Net credit facilities	$ 1,135	1,135
MEC | Line of credit
Line of Credit Facility [Line Items]
Debt to capitalization ratio	0.65
MEC | Commercial paper
Line of Credit Facility [Line Items]
Bank notes	$ 1,500
MEC | Unsecured credit facility, $900 million, expiring June 2022 | Line of credit
Line of Credit Facility [Line Items]
Credit facilities		900
MEC | Unsecured 364-day Credit Facility, $5 million, expiring June | Line of credit
Line of Credit Facility [Line Items]
Credit facilities	$ 5
MEC | Unsecured credit facility, $600 million, expiring August 2021 | Line of credit
Line of Credit Facility [Line Items]
Credit facilities		$ 600