Pension and Other Postretirement Benefits - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 18	$ 18	$ 53	$ 42
Interest cost	62	44	183	81
Expected return on plan assets	(101)	(80)	(298)	(140)
Recognized net actuarial loss	47	36	139	55
Amortization of prior service cost (benefit)	2	2	6	5
Settlements	3	13	3	13
Special termination benefits			3	1
Net periodic benefit cost	31	33	89	57
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	2	1	4	1
Interest cost	9	6	28	8
Recognized net actuarial loss	3	3	10	3
Amortization of prior service cost (benefit)	(1)	(2)	(4)	(2)
Net periodic benefit cost	$ 13	$ 8	$ 38	$ 10