Employee Benefits - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 44	$ 47
Interest cost	52	48
Expected return on plan assets	(66)	(56)
Prior service cost (credit) amortization	(1)	(1)
Actuarial loss (gain) amortization	44	59
Net periodic benefit cost	73	97
Postretirement Welfare Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	1	1
Prior service cost (credit) amortization	(1)	(1)
Actuarial loss (gain) amortization	(1)	(1)
Net periodic benefit cost	$ (1)	$ (1)