Restructuring Charges - Restructuring Roll Forward (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Restructuring Reserve [Roll Forward]
Restructuring Reserve	$ 114	$ 54
Liability recognized	116	211	$ 129
Payments	(141)	(129)
Other adjustments	(17)	(22)
Restructuring Reserve	72	114	54
Employee Severance [Member]
Restructuring Reserve [Roll Forward]
Restructuring Reserve	113	54
Liability recognized	57	177
Payments	(102)	(116)
Other adjustments	(1)	(2)
Restructuring Reserve	67	113	54
Other Restructuring [Member]
Restructuring Reserve [Roll Forward]
Restructuring Reserve	1	0
Liability recognized	59	34
Payments	(39)	(13)
Other adjustments	(16)	(20)
Restructuring Reserve	$ 5	$ 1	$ 0