Pension Plans and Other Postretirement Benefits (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Pension Plans, Defined Benefit [Member]
Components of net periodic benefit cost
Service cost	$ 30	$ 25
Interest cost	25	25
Expected return on plan assets	(27)	(27)
Amortization of net actuarial loss (gain)	13	9
Net periodic benefit cost	41	32
Other Postretirement Benefit Plans, Defined Benefit [Member]
Components of net periodic benefit cost
Service cost	3	2
Interest cost	4	4
Expected return on plan assets	0	0
Amortization of net actuarial loss (gain)	0	(2)
Net periodic benefit cost	$ 7	$ 4