Other Expenses (Restructuring Charges) (Details) - Unit Cost Initiative - Other expenses - Severance - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Restructuring Cost and Reserve [Line Items]
Balance, beginning of period	$ 19	$ 38	$ 22	$ 35
Restructuring charges	9	11	18	22
Cash payments	(8)	(32)	(20)	(40)
Balance, end of period	20	17	20	17
Total restructuring charges incurred since inception of initiative	$ 91	$ 57	$ 91	$ 57