Restructuring - Schedule of Restructuring Balances and Activity (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Restructuring Reserve
Liability at beginning of period	$ 333	$ 467	$ 504
Payments	(103)	(144)	(127)
Additional expenses and other	7	10	90
Liability at end of period	$ 237	$ 333	$ 467