Pension and Other Postretirement Benefits - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 20	$ 23
Interest cost	55	58
Expected return on plan assets	(77)	(83)
Recognized net actuarial loss	42	55
Amortization of prior service cost (benefit)	1	1
Curtailments	5	0
Net periodic benefit cost	46	54
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	2	2
Interest cost	7	8
Recognized net actuarial loss	2	1
Amortization of prior service cost (benefit)	(2)	(2)
Net periodic benefit cost	$ 9	$ 9