Employee Benefits - Components of Net Periodic Benefit Cost (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 38	$ 42	$ 76	$ 84
Interest cost	49	43	98	85
Expected return on plan assets	(52)	(44)	(104)	(88)
Prior service cost (credit) and transition obligation (asset) amortization	(1)	(2)	(2)	(3)
Actuarial loss (gain) amortization	39	66	79	132
Net periodic benefit cost	73	105	147	210
Postretirement Welfare Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost		1		2
Interest cost	1	1	2	2
Expected return on plan assets		(1)		(1)
Prior service cost (credit) and transition obligation (asset) amortization	(1)		(2)
Actuarial loss (gain) amortization	(1)	(2)	(3)	(5)
Net periodic benefit cost	$ (1)	$ (1)	$ (3)	$ (2)