Pension and Other Postretirement Benefits - Periodic Benefit Cost (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Pension
Components of net periodic benefit cost:
Service cost	$ 13	$ 14	$ 40	$ 41
Interest cost	10	15	30	45
Expected return on plan assets	(26)	(28)	(77)	(84)
Amortization of actuarial loss (gain)	14	11	40	35
Amortization of prior service cost	0	1	1	1
Total net periodic benefit cost	11	13	34	38
Other Postretirement Benefits
Components of net periodic benefit cost:
Service cost	2	2	6	6
Interest cost	3	4	8	12
Expected return on plan assets	(7)	(6)	(20)	(18)
Amortization of actuarial loss (gain)	0	0	0	0
Amortization of prior service cost	0	0	0	0
Total net periodic benefit cost	$ (2)	$ 0	$ (6)	$ 0