Employee Benefits - Components of Net Periodic Benefit Cost (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 38	$ 42	$ 114	$ 126
Interest cost	50	43	148	128
Expected return on plan assets	(51)	(44)	(155)	(132)
Prior service cost (credit) and transition obligation (asset) amortization	(2)	(1)	(4)	(4)
Actuarial loss (gain) amortization	39	66	118	198
Net periodic benefit cost	74	106	221	316
Postretirement Welfare Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost		1		3
Interest cost	1	1	3	3
Expected return on plan assets	(1)		(1)	(1)
Prior service cost (credit) and transition obligation (asset) amortization	(1)		(3)
Actuarial loss (gain) amortization	(1)	(3)	(4)	(8)
Net periodic benefit cost	$ (2)	$ (1)	$ (5)	$ (3)