Employee Benefits - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 47	$ 38	$ 94	$ 76
Interest cost	49	49	97	98
Expected return on plan assets	(55)	(52)	(111)	(104)
Prior service cost (credit) amortization	(1)	(1)	(2)	(2)
Actuarial loss (gain) amortization	58	39	117	79
Net periodic benefit cost	98	73	195	147
Postretirement Welfare Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	0		0
Interest cost	1	1	2	2
Expected return on plan assets	(1)		(1)
Prior service cost (credit) amortization		(1)	(1)	(2)
Actuarial loss (gain) amortization	(1)	(1)	(2)	(3)
Net periodic benefit cost	$ (1)	$ (1)	$ (2)	$ (3)