Pension and Other Postretirement Benefits - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Defined Benefit Plan Disclosure [Line Items]
Settlements	$ 3	$ 0
Pension benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	1	7
Interest cost	12	47
Expected return on plan assets	(23)	(70)
Recognized net actuarial loss	14	42
Settlements	3	0
Net periodic benefit cost	7	26
Discontinued operations	0	20
Net periodic benefit cost	7	6
Other postretirement benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	0	1
Interest cost	1	6
Recognized net actuarial loss	0	2
Amortization of prior service benefit	(1)	(2)
Net periodic benefit cost	0	7
Discontinued operations	0	6
Net periodic benefit cost	$ 0	$ 1