Debt - Letters of Credit, Borrowings and Repayments (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Debt
Outstanding letters of credit	$ 155		$ 184
Credit facilities and commercial paper program
Debt
Total borrowings	500	$ 10,500
Total repayments	$ 500	$ 10,700