Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 29	$ 31	$ 84	$ 93
Interest cost	50	50	151	150
Expected return on plan assets	(54)	(49)	(162)	(147)
Amortization of unrecognized (gain) loss and prior service cost (credit)	19	21	57	65
Settlement loss	2	1	4	2
Total	46	54	134	163
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	4	5	14	15
Interest cost	22	20	66	61
Expected return on plan assets			(1)	(1)
Amortization of unrecognized (gain) loss and prior service cost (credit)	(13)	(13)	(40)	(40)
Curtailment gain	(47)		(47)
Total	$ (34)	$ 12	$ (8)	$ 35