Retirement and Postretirement Benefit Plans - Components of Net Periodic Benefit Costs (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Net Periodic Benefit Costs
Employer contributions	$ 91	$ 1
Pension Benefit
Net Periodic Benefit Costs
Settlement gain	9
Curtailment gain	5
Special termination benefits	16
Net periodic benefit cost	11	2
Postretirement Benefit
Net Periodic Benefit Costs
Net periodic benefit cost	$ 20	$ 14