Pension Plans and Other Postretirement Benefits - Components of Net Periodic Benefit Cost Table (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Pension Plan [Member]
Components of net periodic benefit cost
Service cost	$ 24	$ 30	$ 73	$ 89
Interest cost	24	25	73	76
Expected return on plan assets	(24)	(27)	(75)	(82)
Amortization of net actuarial loss (gain)	12	13	30	39
Amortization of net prior service cost (credit)	0	0	0	0
Settlement expense	102	0	126	0
Termination benefits expense			44	0
Curtailment expense			8	0
Net periodic benefit cost	138	41	279	122
Other Postretirement Benefit Plans [Member]
Components of net periodic benefit cost
Service cost	1	3	2	8
Interest cost	3	3	9	11
Expected return on plan assets	0	0	0	0
Amortization of net actuarial loss (gain)	0	0	0	0
Amortization of net prior service cost (credit)	(6)	1	(18)	2
Settlement expense	0	0	0	0
Termination benefits expense			0	0
Curtailment expense			0	0
Net periodic benefit cost	$ (2)	$ 7	$ (7)	$ 21