Employee Pension and Other Postretirement Benefits - Components of Net Periodic Benefit Cost (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 21
Interest cost	24
Expected return on plan assets	(42)
Recognized net actuarial loss	27
Total net periodic benefit cost	30
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	21	21	43	39
Interest cost	24	24	49	50
Expected return on plan assets	(42)	(36)	(83)	(73)
Recognized net actuarial loss	27	21	54	44
Total net periodic benefit cost	30	30	63	60
Other Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	3	3	7	6
Interest cost	3	3	6	6
Expected return on plan assets	(4)	(3)	(8)	(6)
Recognized net actuarial loss	2	2	3	4
Total net periodic benefit cost	4	5	8	10
Superannuation, SERP and ESBP [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	1	1	2	2
Recognized net actuarial loss			1	1
Total net periodic benefit cost	$ 1	$ 1	$ 3	$ 3