Pension and Other Postretirement Benefits - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Defined Benefit Plan Disclosure [Line Items]
Settlements	$ 3	$ 4	$ 67	$ (8)
Pension benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	1	6	10	19
Interest cost	17	59	81	177
Expected return on plan assets	(21)	(71)	(115)	(214)
Recognized net actuarial loss	12	35	66	104
Amortization of prior service cost (benefit)	0	0	0	1
Settlements	8	4	72	8
Net periodic benefit cost	17	33	114	95
Discontinued operations	0	21	20	69
Net periodic benefit cost	17	12	94	26
Other postretirement benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	1	1	3	5
Interest cost	2	7	9	21
Recognized net actuarial loss	0	1	2	3
Amortization of prior service cost (benefit)	(2)	(1)	(5)	(4)
Curtailments	(2)	0	(2)	(58)
Net periodic benefit cost	(1)	8	7	(33)
Discontinued operations	0	7	6	(22)
Net periodic benefit cost	$ (1)	$ 1	$ 1	$ (11)