Retirement and Postretirement Benefit Plans - Components of Net Periodic Benefit Costs (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Net Periodic Benefit Costs
Net gains related to pension settlement and curtailment	$ 6	$ 14
Net periodic benefit costs related to pension special termination benefits	0	16
Contributions to qualified and supplemental pension plans	147	91
Pension Benefit
Net Periodic Benefit Costs
Net periodic benefit costs (gains)	(5)	12
Postretirement Benefit
Net Periodic Benefit Costs
Net periodic benefit costs (gains)	$ 20	$ 20