Pension and Other Postretirement Benefits - Additional Information (Details) $ in Millions		3 Months Ended				12 Months Ended
	Apr. 13, 2018 employee	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Curtailments				$ 9
Offset to AOCI, net of tax						$ 255	$ (220)	$ (482)
Duration of labor agreement	5 years
Number of employees included in Labor Agreement | employee	1,300
Employees subject to retirement benefit increase under Labor Agreement | employee	300
Restructuring and other charges						9	165	155
Pension benefits
Defined Benefit Plan Disclosure [Line Items]
Curtailments						23	0	0
Service cost						46	90	155
Other postretirement benefits
Defined Benefit Plan Disclosure [Line Items]
Decrease in pension liability, plan amendment			$ 32
Curtailments		$ 9	(28)			(28)	0	0
Offset to AOCI, net of tax		13	$ 4
Service cost						7	$ 7	13
United States | Pension benefits
Defined Benefit Plan Disclosure [Line Items]
Decrease in pension liability, plan amendment					$ 136
Curtailments					5
Offset to AOCI, net of tax					$ 141
Pension Settlement Cost
Defined Benefit Plan Disclosure [Line Items]
Restructuring and other charges		$ 96				$ 96		$ 2