Retirement and Postretirement Benefit Plans - Components of Net Periodic Benefit Costs (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Net Periodic Benefit Costs
Special termination benefits	$ 118		$ 116
Employer contributions	11		102
Pension Benefit
Net Periodic Benefit Costs
Settlement gain	118		116
Curtailment gain	118		116
Net periodic benefit cost	13		24
Postretirement Benefit
Net Periodic Benefit Costs
Net periodic benefit cost	$ 18	$ 15	$ 38	$ 29