Pension Plans and Other Postretirement Benefits (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Pension Plans, Defined Benefit [Member]
Components of net periodic benefit cost
Service cost	$ 30	$ 25	$ 89	$ 74
Interest cost	25	25	76	75
Expected return on plan assets	(27)	(27)	(82)	(80)
Amortization of net actuarial loss (gain)	13	9	39	26
Amortization of net prior service cost (credit)	0	0	0	0
Net periodic benefit cost	41	32	122	95
Other Postretirement Benefit Plans, Defined Benefit [Member]
Components of net periodic benefit cost
Service cost	3	1	8	5
Interest cost	3	4	11	11
Expected return on plan assets	0	0	0	0
Amortization of net actuarial loss (gain)	0	(2)	0	(5)
Amortization of net prior service cost (credit)	1	0	2	0
Net periodic benefit cost	$ 7	$ 3	$ 21	$ 11