Pension and Other Postretirement Benefits - Periodic Benefit Cost (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Pension
Components of net periodic benefit cost:
Service cost	$ 14	$ 13	$ 27	$ 27
Interest cost	10	15	20	30
Expected return on plan assets	(26)	(28)	(51)	(56)
Amortization of actuarial loss (gain)	13	12	26	24
Amortization of prior service cost	1	0	1	0
Total net periodic benefit cost	12	12	23	25
Other Postretirement Benefits
Components of net periodic benefit cost:
Service cost	2	2	4	4
Interest cost	2	4	5	8
Expected return on plan assets	(6)	(6)	(13)	(12)
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	$ (4)	$ 0