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, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 9	$ 13	$ 21	$ 26
Interest cost	28	25	55	50
Expected return on plan assets	(36)	(36)	(71)	(73)
Amortization of prior service cost (benefit)	1	1	1	1
Amortization of net actuarial loss	11	18	22	34
Net periodic benefit cost	13	21	28	38
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	2	2	4	4
Interest cost	6	8	13	16
Expected return on plan assets	(6)	(5)	(12)	(10)
Amortization of prior service cost (benefit)	(4)	(1)	(7)	(2)
Amortization of net actuarial loss	(1)	4	2	8
Net periodic benefit cost	$ (3)	$ 8	$ 0	$ 16