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
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 13	$ 7	$ 26	$ 18
Interest cost	25	27	50	53
Expected return on plan assets	(36)	(32)	(73)	(66)
Amortization of prior service cost (benefit)	1	1	1	1
Amortization of net actuarial loss	18	18	34	32
Net periodic benefit cost	21	21	38	38
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	2	3	4	4
Interest cost	8	8	16	17
Expected return on plan assets	(5)	(4)	(10)	(9)
Amortization of prior service cost (benefit)	(1)	(1)	(2)	(2)
Amortization of net actuarial loss	4	2	8	7
Termination benefits		1		1
Net periodic benefit cost	$ 8	$ 9	$ 16	$ 18