Employee Benefits - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 45	$ 47	$ 133	$ 141
Interest cost	53	49	158	146
Expected return on plan assets	(66)	(56)	(198)	(167)
Prior service cost (credit) amortization	(2)	(2)	(4)	(4)
Actuarial loss (gain) amortization	44	59	131	176
Net periodic benefit cost	74	97	220	292
Postretirement Welfare Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost		1	2	3
Expected return on plan assets	(1)		(1)	(1)
Prior service cost (credit) amortization		(1)	(2)	(2)
Actuarial loss (gain) amortization	(1)	(1)	(3)	(3)
Net periodic benefit cost	$ (2)	$ (1)	$ (4)	$ (3)