Restructuring Costs - Restructuring Liability (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Restructuring Reserve [Roll Forward]
Restructuring costs	$ 0	$ 31	$ 0	$ 85
Termination benefits [Member]
Restructuring Reserve [Roll Forward]
Restructuring costs		6		19
Professional services and other [Member]
Restructuring Reserve [Roll Forward]
Restructuring costs		25		66
Operational Improvement Program [Member]
Restructuring Reserve [Roll Forward]
Restructuring liability, beginning balance			21	25	$ 25	$ 26	$ 11	$ 0
Restructuring costs		$ 31		85	134	145	126	36
Cash payments			(17)		(138)	(146)	(111)	(25)
Restructuring liability, ending balance	4		4		21	25	26	11
Operational Improvement Program [Member] | Termination benefits [Member]
Restructuring Reserve [Roll Forward]
Restructuring liability, beginning balance			14	7	7	15	5	0
Restructuring costs					48	23	36	16
Cash payments			(11)		(41)	(31)	(26)	(11)
Restructuring liability, ending balance	3		3		14	7	15	5
Operational Improvement Program [Member] | Professional services and other [Member]
Restructuring Reserve [Roll Forward]
Restructuring liability, beginning balance			7	18	18	11	6	0
Restructuring costs					86	122	90	20
Cash payments			(6)		(97)	(115)	(85)	(14)
Restructuring liability, ending balance	1		1		7	18	11	$ 6
Business Restructure Program {Member]
Restructuring Reserve [Roll Forward]
Restructuring liability, beginning balance			1	26	26	0
Restructuring costs					(2)	48
Cash payments			(1)		(23)	(22)
Restructuring liability, ending balance	0		0		1	26	0
Business Restructure Program {Member] | Termination benefits [Member]
Restructuring Reserve [Roll Forward]
Restructuring liability, beginning balance			1	26	26	0
Restructuring costs					(2)	45
Cash payments			(1)		(23)	(19)
Restructuring liability, ending balance	0		0		1	26	0
Business Restructure Program {Member] | Professional services and other [Member]
Restructuring Reserve [Roll Forward]
Restructuring liability, beginning balance			0	$ 0	0	0
Restructuring costs					0	3
Cash payments			0		0	(3)
Restructuring liability, ending balance	$ 0		$ 0		$ 0	$ 0	$ 0