Pension and Other Postretirement Benefit Programs - Net Periodic Benefit Cost Relating to Pension and Other Postemployment Employee Benefit Plans (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Pension benefits
Net periodic benefit cost
Service cost	$ 21	$ 18
Interest cost	24	47
Expected return on plan assets	(41)	(72)
Amortization of net losses and prior service costs	19	15
Net periodic cost	23	8
OPEB
Net periodic benefit cost
Interest cost	1	2
Amortization of net losses and prior service costs	(4)	(7)
Net periodic cost	$ (3)	$ (5)