Pension Plans and Other Postretirement Benefits Components of Net Periodic Benefit Cost Table (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Pension Plan [Member]
Components of net periodic benefit cost
Service cost	$ 26	$ 30
Interest cost	26	25
Expected return on plan assets	(27)	(27)
Amortization of net actuarial loss (gain)	8	13
Amortization of net prior service cost (credit)	0	0
Termination benefits expense	44	0
Curtailment expense	8	0
Net periodic benefit cost	85	41
Other Postretirement Benefit Plans, Defined Benefit [Member]
Components of net periodic benefit cost
Service cost	1	3
Interest cost	3	4
Expected return on plan assets	0	0
Amortization of net actuarial loss (gain)	0	0
Amortization of net prior service cost (credit)	(6)	0
Termination benefits expense	0	0
Curtailment expense	(3)	0
Net periodic benefit cost	$ (5)	$ 7