Employee Benefits - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 44	$ 47	$ 88	$ 94
Interest cost	53	49	105	97
Expected return on plan assets	(66)	(55)	(132)	(111)
Prior service cost (credit) amortization	(1)	(1)	(2)	(2)
Actuarial loss (gain) amortization	43	58	87	117
Net periodic benefit cost	73	98	146	195
Postretirement Welfare Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	1	1	2	2
Expected return on plan assets		(1)		(1)
Prior service cost (credit) amortization	(1)		(2)	(1)
Actuarial loss (gain) amortization	(1)	(1)	(2)	(2)
Net periodic benefit cost	$ (1)	$ (1)	$ (2)	$ (2)