Other Expenses (Restructuring Charges) (Details) - Unit Cost Initiative - Other expenses - Severance - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Restructuring Cost and Reserve [Line Items]
Balance, beginning of period	$ 22	$ 35
Restructuring charges	9	11
Cash payments	(12)	(8)
Balance, end of period	19	38
Total restructuring charges incurred since inception of initiative	$ 82	$ 46