Pension Plans and Other Postretirement Benefits (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Pension Plans, Defined Benefit [Member]
Components of net periodic benefit cost
Service cost	$ 29	$ 24	$ 59	$ 49
Interest cost	26	25	51	50
Expected return on plan assets	(28)	(26)	(55)	(53)
Amortization of net actuarial loss (gain)	13	8	26	17
Amortization of net prior service cost (credit)	0	0	0	0
Net periodic benefit cost	40	31	81	63
Other Postretirement Benefit Plans, Defined Benefit [Member]
Components of net periodic benefit cost
Service cost	2	2	5	4
Interest cost	4	3	8	7
Expected return on plan assets	0	0	0	0
Amortization of net actuarial loss (gain)	0	(1)	0	(3)
Amortization of net prior service cost (credit)	1	0	1	0
Net periodic benefit cost	$ 7	$ 4	$ 14	$ 8