Net Periodic Benefit Cost Relating to Pension and Other Postemployment Benefit Plans (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Pension benefits
Net periodic benefit cost
Service cost	$ 22	$ 22
Interest cost	46	45
Expected return on plan assets	(78)	(72)
Amortization of net losses and other deferred amounts	24	40
Net periodic benefit cost	14	35
OPEB
Net periodic benefit cost
Interest cost	2	2
Amortization of net loss and prior service credit	(6)	(6)
Net periodic benefit cost	$ (4)	$ (4)