Pension Plans and Other Postretirement Benefits - Net Periodic Benefit Costs (Details) - Pension Plan - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Interest cost	$ 168	$ 225	$ 25
Expected return on plan assets	(388)	(357)	(33)
Amortization of net actuarial loss	2	1	2
Net periodic benefit cost (income)	$ (218)	$ (131)	$ (6)