Short-Term Debt (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Short-term Debt [Line Items]
Amount outstanding at period end	$ 1,747	$ 584
Commercial Paper
Short-term Debt [Line Items]
Borrowing limit	4,300	3,100
Amount outstanding at period end	1,747	584
Average amount outstanding	1,742	1,126
Maximum amount outstanding	$ 1,857	$ 2,080
Weighted average interest rate, computed on a daily basis	0.57%	1.45%
Weighted average interest rate at period end	0.56%	0.23%