Other Expenses (Restructuring Charges) (Details) - Unit Cost Initiative - Other expenses - Severance - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Restructuring Cost and Reserve [Line Items]
Balance, beginning of period	$ 20	$ 17	$ 22	$ 35
Restructuring charges	17	3	35	25
Cash payments	(17)	(3)	(37)	(43)
Balance, end of period	20	17	20	17
Total restructuring charges incurred since inception of initiative	$ 108	$ 60	$ 108	$ 60