Short-Term Debt (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Short-term Debt [Line Items]
Amount outstanding at period end	$ 1,765	$ 595
Commercial Paper
Short-term Debt [Line Items]
Borrowing limit	4,300	3,600
Amount outstanding at period end	1,765	595
Average amount outstanding	1,404	1,115
Maximum amount outstanding	$ 2,080	$ 1,780
Weighted average interest rate, computed on a daily basis	2.05%	2.72%
Weighted average interest rate at period end	1.96%	2.34%