Retirement and Postretirement Benefit Plans - Components of Net Periodic Benefit Costs (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Net Periodic Benefit Costs
Net gains related to pension settlement and curtailment	$ 1	$ 8	$ 11	$ 142
Net periodic benefit costs related to pension special termination benefits	0	3	0	21
Contributions to qualified and supplemental pension plans	5	30	157	132
Pension Benefit
Net Periodic Benefit Costs
Net periodic benefit costs (gains)	(4)	(2)	(12)	22
Postretirement Benefit
Net Periodic Benefit Costs
Net periodic benefit costs (gains)	$ 23	$ 18	$ 61	$ 56