Restructuring - Schedule of Restructuring Balances and Activity (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Restructuring Reserve
Liability at beginning of period	$ 237	$ 333	$ 467
Reclassified to lease liability	(195)	0	0
Payments	(5)	(103)	(144)
Additional expenses and other	1	7	10
Liability at end of period	$ 38	$ 237	$ 333