Retirement and Postretirement Benefit Plans - Components of Net Periodic Benefit Costs (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Net Periodic Benefit Costs
Net gains related to pension settlement and curtailment	$ 4	$ 120	$ 10	$ 134
Net periodic benefit costs related to pension special termination benefits	0	2	0	18
Contributions to qualified and supplemental pension plans	5	11	152	102
Pension Benefit
Net Periodic Benefit Costs
Net periodic benefit costs (gains)	(3)	13	(8)	24
Postretirement Benefit
Net Periodic Benefit Costs
Net periodic benefit costs (gains)	$ 18	$ 18	$ 38	$ 38