Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 49	$ 27	$ 98	$ 55
Interest cost	55	50	110	101
Expected return on plan assets	(61)	(54)	(121)	(108)
Amortization of unrecognized (gain) loss and prior service cost (credit)	32	20	63	38
Settlement loss	1	1	2	2
Total	76	44	152	88
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	4	6	6	10
Interest cost	17	22	34	44
Expected return on plan assets	(1)	(1)	(1)	(1)
Amortization of unrecognized (gain) loss and prior service cost (credit)	(18)	(14)	(36)	(27)
Total	$ 2	$ 13	$ 3	$ 26