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		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 2,747	$ 2,649	$ 5,606	$ 5,370
Interest cost	5,626	5,989	11,215	11,896
Expected return on plan assets	(10,722)	(10,666)	(21,561)	(21,497)
Amortization of prior service cost/(credit)	781	708	1,561	1,412
Amortization of unrecognized/actuarial loss (gain)	3,523	3,311	6,913	6,133
Total net periodic benefit cost	1,955	1,991	3,734	3,314
Other Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	238	284	545	579
Interest cost	410	513	861	998
Amortization of prior service cost/(credit)	(46)	(46)	(91)	(91)
Amortization of unrecognized/actuarial loss (gain)	(272)	(46)	(345)	(189)
Total net periodic benefit cost	$ 330	$ 705	$ 970	$ 1,297