Retirement Plans and Other Post-Retirement Benefits - Schedule of Net Periodic Costs of Pension and Post Retirement Medical Benefit Plans (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 2,721	$ 2,730
Interest cost	5,907	6,087
Expected return on plan assets	(10,831)	(11,386)
Amortization of prior service cost/(credit)	704	674
Amortization of unrecognized/actuarial loss (gain)	2,822	3,054
Total net periodic benefit cost	1,323	1,159
Other Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	295	323
Interest cost	485	540
Amortization of prior service cost/(credit)	(45)	(45)
Amortization of unrecognized/actuarial loss (gain)	(143)	(51)
Total net periodic benefit cost	$ 592	$ 767