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, 2018	Mar. 31, 2017
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 2,859	$ 2,721
Interest cost	5,589	5,907
Expected return on plan assets	(10,839)	(10,831)
Amortization of prior service cost/(credit)	780	704
Amortization of unrecognized/actuarial loss (gain)	3,390	2,822
Total net periodic benefit cost	1,779	1,323
Other Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	307	295
Interest cost	451	485
Amortization of prior service cost/(credit)	(45)	(45)
Amortization of unrecognized/actuarial loss (gain)	(73)	(143)
Total net periodic benefit cost	$ 640	$ 592