Restructuring and Other Charges - Activity and Reserve Balances for Restructuring Charges (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
Restructuring Reserve [Roll Forward]
Restructuring reserve beginning balance			$ 58	$ 59	$ 59
Cash payments			(38)		(65)
Restructuring charges			21		65
Other			0		(1)
Restructuring reserve ending balance	$ 41		41		58
Restructuring and other charges	(2)	$ 19	20	118
Layoff costs
Restructuring Reserve [Roll Forward]
Restructuring reserve beginning balance			56	50	50
Cash payments			(37)		(59)
Restructuring charges			8		64
Other			0		1
Restructuring reserve ending balance	27		27		56
Other exit costs
Restructuring Reserve [Roll Forward]
Restructuring reserve beginning balance			2	$ 9	9
Cash payments			(1)		(6)
Restructuring charges			13		1
Other			0		(2)
Restructuring reserve ending balance	$ 14		14		2
Postretirement benefits
Restructuring Reserve [Roll Forward]
Restructuring and other charges			28
Pension costs
Restructuring Reserve [Roll Forward]
Restructuring and other charges			$ 18
Stock awards reversal
Restructuring Reserve [Roll Forward]
Restructuring and other charges					$ 13