Short-Term and Long-Term Debt (Credit Facilities and Letters of Credit) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($)
Line of Credit Facility [Line Items]
Credit Facilities	Total
Maximum Available	$ 4,194
LOCs issued	$ 2,116
Five-year revolving credit facility [Member]
Line of Credit Facility [Line Items]
Credit Facilities	Five-year revolving credit facility
Expiration Date	Jul. 31, 2024
Maximum Available	$ 2,250
LOCs issued	$ 219
LOC facility due August 2031 [Member]
Line of Credit Facility [Line Items]
Credit Facilities	LOC facility (1)
Expiration Date	Aug. 26, 2031
Maximum Available	$ 990
LOCs issued	$ 943
LOC facility due October 2031 [Member]
Line of Credit Facility [Line Items]
Credit Facilities	LOC facility (1)
Expiration Date	Oct. 01, 2031
Maximum Available	$ 954
LOCs issued	$ 954