Net Periodic Benefit Cost Relating to Pension and Other Postemployment Benefit Plans (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Pension benefits
Net periodic benefit cost
Service cost	$ 25	$ 23	$ 47	$ 45
Interest cost	45	45	91	90
Expected return on plan assets	(79)	(73)	(157)	(145)
Amortization of net losses and other deferred amounts	24	41	48	81
Net periodic benefit cost	15	36	29	71
OPEB
Net periodic benefit cost
Interest cost	2	2	4	4
Amortization of net loss and prior service credit	(6)	(7)	(12)	(13)
Net periodic benefit cost	$ (4)	$ (5)	$ (8)	$ (9)