Other Expenses (Restructuring Charges) (Details) - Other expenses - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Restructuring Charges
Balance at January 1,	$ 2	$ 22	$ 37
Restructuring charges		1	64
Cash payments		(21)	(79)
Balance at December 31,		2	22
Total restructuring charges incurred since inception of initiative		430	429
Severance
Restructuring Charges
Balance at January 1,	1	18	31
Restructuring charges		0	60
Cash payments		(17)	(73)
Balance at December 31,		1	18
Total restructuring charges incurred since inception of initiative		383	383
Lease and Asset Impairment
Restructuring Charges
Balance at January 1,	1	4	6
Restructuring charges		1	4
Cash payments		(4)	(6)
Balance at December 31,		1	4
Total restructuring charges incurred since inception of initiative		47	46
Unit Cost Initiative | Severance
Restructuring Charges
Balance at January 1,	35	0
Restructuring charges	38	35
Cash payments	(51)	0
Balance at December 31,	22	35	$ 0
Total restructuring charges incurred since inception of initiative	$ 73	$ 35