Pension and Other Postretirement Benefits - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	3 Months Ended				12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Curtailments			$ 9
Pension benefits paid
Defined Benefit Plan Disclosure [Line Items]
Service cost					$ 46	$ 90	$ 155
Interest cost					219	234	431
Expected return on plan assets					(306)	(332)	(677)
Recognized net actuarial loss					168	220	380
Amortization of prior service cost (benefit)					3	5	13
Settlements					96	0	19
Curtailments					23	0	0
Special termination benefits					0	0	2
Net periodic benefit cost					249	217	323
Discontinued operations					0	0	122
Net amount recognized in Statement of Consolidated Operations					249	217	201
Pension benefits paid | United States
Defined Benefit Plan Disclosure [Line Items]
Curtailments				$ 5
Net periodic benefit cost					239	206	261
Other postretirement benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost					7	7	13
Interest cost					28	30	63
Expected return on plan assets					0	0	0
Recognized net actuarial loss					7	5	24
Amortization of prior service cost (benefit)					(7)	(8)	(24)
Settlements					0	0	0
Curtailments	$ 9	$ (28)			(28)	0	0
Special termination benefits					0	0	0
Net periodic benefit cost					7	34	76
Discontinued operations					0	0	41
Net amount recognized in Statement of Consolidated Operations					7	34	35
Decrease in net periodic benefit cost for the recognition of the federal subsidy awarded under Medicare Part D					$ 10	$ 11	$ 22