Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 49	$ 28
Interest cost	55	51
Expected return on plan assets	(60)	(54)
Amortization of unrecognized (gain) loss and prior service cost (credit)	31	18
Settlement loss	1	1
Total	76	44
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	2	4
Interest cost	17	22
Amortization of unrecognized (gain) loss and prior service cost (credit)	(18)	(13)
Total	$ 1	$ 13