Other Expenses (Restructuring Charges) (Details) - Other expenses - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Restructuring Cost and Reserve [Line Items]
Balance at January 1,		$ 2	$ 22
Restructuring charges			1
Cash payments			(21)
Balance at December 31,			2
Total restructuring charges incurred since inception of initiative			430
Severance
Restructuring Cost and Reserve [Line Items]
Balance at January 1,		1	18
Restructuring charges			0
Cash payments			(17)
Balance at December 31,			1
Total restructuring charges incurred since inception of initiative			383
Lease and Asset Impairment
Restructuring Cost and Reserve [Line Items]
Balance at January 1,		1	4
Restructuring charges			1
Cash payments			(4)
Balance at December 31,			1
Total restructuring charges incurred since inception of initiative			47
Unit Cost Initiative | Severance
Restructuring Cost and Reserve [Line Items]
Balance at January 1,	$ 22	35	0
Restructuring charges	63	38	35
Cash payments	(62)	(51)	0
Balance at December 31,	23	22	35
Total restructuring charges incurred since inception of initiative	$ 136	$ 73	$ 35