Pension and Other Postretirement Benefits - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Curtailments				$ 9
Pension benefits paid
Defined Benefit Plan Disclosure [Line Items]
Service cost		$ 12	$ 25	46
Interest cost		97	235	219
Expected return on plan assets		(136)	(286)	(306)
Recognized net actuarial loss		78	139	168
Amortization of prior service cost (benefit)		0	2	3
Settlements		76	9	96
Curtailments		0	0	23
Net periodic benefit cost		127	124	249
Discontinued operations		20	95	100
Net amount recognized in Statement of Consolidated Operations		107	29	149
Pension benefits paid | United States
Defined Benefit Plan Disclosure [Line Items]
Curtailments			17	5
Net periodic benefit cost		58	127	239
Other postretirement benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost		3	7	7
Interest cost		10	28	28
Expected return on plan assets		0	0	0
Recognized net actuarial loss		3	4	7
Amortization of prior service cost (benefit)		(6)	(6)	(7)
Settlements		0	0	0
Curtailments	$ 9	(2)	(58)	(28)
Net periodic benefit cost		8	(25)	7
Discontinued operations		6	(15)	12
Net amount recognized in Statement of Consolidated Operations		2	(10)	(5)
Decrease in net periodic benefit cost for the recognition of the federal subsidy awarded under Medicare Part D		$ 1	$ 11	$ 10