Restructuring and Other Charges - Activity and Reserve Balances for Restructuring Charges (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Restructuring Reserve [Roll Forward]
Restructuring reserve beginning balance	$ 25	$ 58	$ 58
Cash payments	(17)		(49)
Restructuring charges	11		124
Other	48		(108)
Restructuring reserve ending balance	67		25
Restructuring and other charges	(12)	(7)
Layoff costs
Restructuring Reserve [Roll Forward]
Restructuring reserve beginning balance	10	56	56
Cash payments	(14)		(47)
Restructuring charges	11		111
Other	57		(110)
Restructuring reserve ending balance	64		10
Other exit costs
Restructuring Reserve [Roll Forward]
Restructuring reserve beginning balance	15	2	2
Cash payments	(3)		(2)
Restructuring charges	0		13
Other	(9)		2
Restructuring reserve ending balance	3		15
Restructuring and other charges		$ 4
Elimination Of Life Insurance Benefits
Restructuring Reserve [Roll Forward]
Restructuring and other charges	58
Pension Settlement Cost
Restructuring Reserve [Roll Forward]
Restructuring and other charges	(2)
Other Exit Costs, Reclassification Of Lease Exit Costs
Restructuring Reserve [Roll Forward]
Restructuring and other charges	$ 9
Employee Severance Cost, Reclassification Of Pension Costs
Restructuring Reserve [Roll Forward]
Other			119
Employee Severance Costs, Credit In Post-Retirement Costs
Restructuring Reserve [Roll Forward]
Restructuring and other charges			28
Employee Severance, Reversal Of Prior Restructuring
Restructuring Reserve [Roll Forward]
Restructuring and other charges			$ 19