Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 28	$ 31
Interest cost	51	50
Expected return on plan assets	(54)	(49)
Amortization of unrecognized (gain) loss and prior service cost (credit)	18	22
Settlement loss	1	1
Total	44	55
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	4	5
Interest cost	22	20
Amortization of unrecognized (gain) loss and prior service cost (credit)	(13)	(13)
Total	$ 13	$ 12