Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 48	$ 29	$ 146	$ 84
Interest cost	55	50	165	151
Expected return on plan assets	(61)	(54)	(182)	(162)
Amortization of unrecognized (gain) loss and prior service cost (credit)	32	19	95	57
Settlement loss	3	2	5	4
Total	77	46	229	134
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	4	4	10	14
Interest cost	16	22	50	66
Expected return on plan assets			(1)	(1)
Amortization of unrecognized (gain) loss and prior service cost (credit)	(18)	(13)	(54)	(40)
Curtailment gain		(47)		(47)
Total	$ 2	$ (34)	$ 5	$ (8)