Pension and Other Postretirement Benefits - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 43	$ 44	$ 124	$ 132
Interest cost	114	144	358	433
Expected return on plan assets	(187)	(189)	(558)	(566)
Recognized net actuarial loss	104	117	308	352
Amortization of prior service benefit	4	4	12	12
Settlements	13	13	15	14
Curtailments		9		9
Special termination benefits			1	12
Net periodic benefit cost	91	142	260	398
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	3	4	10	11
Interest cost	16	23	53	69
Recognized net actuarial loss	8	4	19	13
Amortization of prior service benefit	(6)	(9)	(19)	(28)
Curtailments		(5)		(6)
Special termination benefits				1
Net periodic benefit cost	$ 21	$ 17	$ 63	$ 60