Employee Benefits - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 47	$ 38	$ 141	$ 114
Interest cost	49	50	146	148
Expected return on plan assets	(56)	(51)	(167)	(155)
Prior service cost (credit) amortization	(2)	(2)	(4)	(4)
Actuarial loss (gain) amortization	59	39	176	118
Net periodic benefit cost	97	74	292	221
Postretirement Welfare Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	1	1	3	3
Expected return on plan assets		(1)	(1)	(1)
Prior service cost (credit) amortization	(1)	(1)	(2)	(3)
Actuarial loss (gain) amortization	(1)	(1)	(3)	(4)
Net periodic benefit cost	$ (1)	$ (2)	$ (3)	$ (5)