Pension and Other Postretirement Benefits - Additional Information (Details) $ in Millions			1 Months Ended	3 Months Ended		6 Months Ended	12 Months Ended
	Jul. 25, 2019 employee	Apr. 13, 2018 employee	Jun. 30, 2019 employee	Dec. 31, 2018 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Offset to AOCI, net of tax							$ (388)	$ 255	$ (220)
Curtailment (charge) benefit						$ (9)
Number of employees included in Labor Agreement | employee	560	1,300	3,400
Employees subject to retirement benefit increase under Labor Agreement | employee		300
Labor agreement, number of years covered	4 years		3 years
Labor agreement, one time signing bonus							9
Duration of labor agreement		5 years
Restructuring and other charges							620	9	165
Pension benefits
Defined Benefit Plan Disclosure [Line Items]
Curtailment (charge) benefit							0	(23)	0
Other postretirement benefits
Defined Benefit Plan Disclosure [Line Items]
Decrease in pension liability, plan amendment								32
Offset to AOCI, net of tax				$ 13				4
Curtailment (charge) benefit				$ (9)			58	28	$ 0
United States | Pension benefits
Defined Benefit Plan Disclosure [Line Items]
Decrease in pension liability, plan amendment					$ 136		75
Offset to AOCI, net of tax					141
Curtailment (charge) benefit					$ (5)		(17)
Pension Settlement Cost
Defined Benefit Plan Disclosure [Line Items]
Restructuring and other charges							$ 9	$ 96