Pension and Other Postretirement Benefit Programs - Net Periodic Benefit Cost Relating to Pension and Other Postemployment Employee Benefit Plans (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Pension benefits
Net periodic benefit cost
Service cost	$ 18	$ 23	$ 56	$ 70
Interest cost	47	45	137	136
Expected return on plan assets	(74)	(78)	(214)	(235)
Amortization of net losses and prior service costs (credit)	15	24	44	72
Net periodic cost	6	14	23	43
OPEB
Net periodic benefit cost
Interest cost	2	2	6	6
Amortization of net losses and prior service costs (credit)	(7)	(6)	(21)	(18)
Net periodic cost	$ (5)	$ (4)	$ (15)	$ (12)