Retirement and Postretirement Benefit Plans - Components of Net Periodic Benefit Costs (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Net Periodic Benefit Costs
Net gains related to settlement, curtailment, and special termination benefits	$ 5	$ 24	$ 121	$ 24
Contributions to qualified and supplemental plans	30	7	132	8
Pension Benefit
Net Periodic Benefit Costs
Net periodic benefit costs	0	15	22	19
Postretirement Benefit
Net Periodic Benefit Costs
Net periodic benefit costs	$ 18	$ 19	$ 56	$ 48