Pension Plans and Other Postretirement Benefits - Changes in Level 3 Pension Plan Assets (Details) - Pension Plan - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Change in plan assets:
Beginning balance	$ 6,845	$ 6,395
Ending balance	6,677	6,845
Level 3
Change in plan assets:
Beginning balance	343	354
Actual return on plan assets	43	(2)
Purchases, sales and settlements	(8)	(8)
Transfers out of Level 3	0	(1)
Ending balance	378	343
Real estate | Level 3
Change in plan assets:
Beginning balance	343	353
Actual return on plan assets	43	(2)
Purchases, sales and settlements	(8)	(8)
Transfers out of Level 3	0	0
Ending balance	378	343
U.S. corporate securities | Level 3
Change in plan assets:
Beginning balance	0	1
Actual return on plan assets		0
Purchases, sales and settlements		0
Transfers out of Level 3		(1)
Ending balance	$ 0	$ 0