Other Expenses (Restructuring Charges) (Details) - Unit Cost Initiative - Other expenses - Severance - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Restructuring Cost and Reserve [Line Items]
Balance, beginning of period	$ 17	$ 19	$ 23	$ 22
Restructuring charges	14	9	21	18
Cash payments	(11)	(8)	(24)	(20)
Balance, end of period	20	20	20	20
Total restructuring charges incurred since inception of initiative	$ 157	$ 91	$ 157	$ 91