Pension and Other Postretirement Benefits - Components of Net Periodic Benefit Cost (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 14	$ 8	$ 40	$ 26
Interest cost	25	27	75	80
Expected return on plan assets	(36)	(33)	(109)	(99)
Amortization of prior service cost (benefit)		1	1	2
Amortization of net actuarial loss	16	16	50	48
Net periodic benefit cost	19	19	57	57
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	2	2	6	6
Interest cost	6	9	22	26
Expected return on plan assets	(5)	(5)	(15)	(14)
Amortization of prior service cost (benefit)	(4)	(1)	(6)	(3)
Amortization of net actuarial loss	2	4	10	11
Termination benefits				1
Net periodic benefit cost	$ 1	$ 9	$ 17	$ 27