Short-Term Borrowings (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Short-term Debt [Line Items]
Amount outstanding at period end	$ 250	$ 250	$ 0
Money Pool
Short-term Debt [Line Items]
Borrowing limit	100	100	100	$ 100
Amount outstanding at period end	0	0	0	0
Average amount outstanding	81	43	8	29
Maximum amount outstanding	$ 100	$ 100	$ 100	$ 100
Weighted average interest rate, computed on a daily basis (percentage)	0.08%	0.54%	2.42%	1.96%
Commercial Paper
Short-term Debt [Line Items]
Borrowing limit	$ 500	$ 500	$ 500	$ 400
Amount outstanding at period end	250	250	0	42
Average amount outstanding	58	44	72	30
Maximum amount outstanding	$ 250	$ 250	$ 316	$ 144
Weighted average interest rate, computed on a daily basis (percentage)	0.21%	1.11%	2.68%	2.27%
Weighted average interest rate at period end (percentage)	0.29%	0.29%		2.80%