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, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Pension Plan [Member]
Components of net periodic benefit cost
Service cost	$ 23	$ 29	$ 49	$ 59
Interest cost	23	26	49	51
Expected return on plan assets	(24)	(28)	(51)	(55)
Amortization of net actuarial loss (gain)	10	13	18	26
Amortization of net prior service cost (credit)	0	0	0	0
Settlement expense	24	0	24	0
Termination benefits expense			44	0
Curtailment expense	0	0	8	0
Net periodic benefit cost	56	40	141	81
Other Postretirement Benefit Plans [Member]
Components of net periodic benefit cost
Service cost	0	2	1	5
Interest cost	3	4	6	8
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	(12)	1
Settlement expense	0	0	0	0
Termination benefits expense			0	0
Curtailment expense	3	0	0	0
Net periodic benefit cost	$ 0	$ 7	$ (5)	$ 14