Pension Plans and Other Postretirement Benefits - Changes in Level 3 Pension Plan Assets (Details) - Pension Plan - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Change in plan assets:
Beginning balance	$ 6,395	$ 5,663
Ending balance	6,845	6,395
Level 3
Change in plan assets:
Beginning balance	354	430
Actual return on plan assets	(2)	5
Purchases, sales and settlements	(8)	82
Transfers out of Level 3	(1)	1
Ending balance	343	354
Real estate | Level 3
Change in plan assets:
Beginning balance	353	425
Actual return on plan assets	(2)	5
Purchases, sales and settlements	(8)	77
Transfers out of Level 3	0	0
Ending balance	343	353
U.S. corporate securities | Level 3
Change in plan assets:
Beginning balance	1	5
Actual return on plan assets	0	0
Purchases, sales and settlements	0	5
Transfers out of Level 3	(1)	1
Ending balance	$ 0	$ 1