Pension Plans and Other Postretirement Benefits - Components of Net Periodic Benefit Cost Table (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Pension Benefits [Member]
Components of net periodic benefit cost
Service cost	$ 21	$ 23	$ 42	$ 49
Interest cost	21	23	42	49
Expected (return) loss on plan assets	(21)	(24)	(42)	(51)
Amortization of net actuarial loss (gain)	7	10	13	18
Amortization of net prior service cost (credit)	0	0	0	0
Settlement expense	55	24	58	24
Termination benefits expense			4	44
Curtailment expense	0	0	0	8
Net periodic benefit cost	83	56	117	141
Other Benefits [Member]
Components of net periodic benefit cost
Service cost	1	0	1	1
Interest cost	3	3	6	6
Expected (return) loss 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)	(6)	(12)	(12)
Settlement expense	0	0	0	0
Termination benefits expense			0	0
Curtailment expense	0	3	0	0
Net periodic benefit cost	$ (2)	$ 0	$ (5)	$ (5)