Pension and Other Postretirement Benefits - Periodic Benefit Cost (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Pension
Components of net periodic benefit cost:
Service cost	$ 13	$ 13	$ 27	$ 26
Interest cost	15	19	30	39
Expected return on plan assets	(28)	(31)	(56)	(61)
Amortization of actuarial loss (gain)	12	6	24	11
Amortization of prior service cost	0	1	0	1
Total net periodic benefit cost	12	8	25	16
Other Postretirement Benefits
Components of net periodic benefit cost:
Service cost	2	2	4	4
Interest cost	4	5	8	10
Expected return on plan assets	(6)	(5)	(12)	(11)
Amortization of actuarial loss (gain)	0	(1)	0	(1)
Amortization of prior service cost	0	0	0	0
Total net periodic benefit cost	$ 0	$ 1	$ 0	$ 2