Pension and Other Postretirement Benefits - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 90	$ 155	$ 175
Interest cost	234	431	577
Expected return on plan assets	(332)	(677)	(753)
Recognized net actuarial loss	220	380	468
Amortization of prior service cost (benefit)	5	13	16
Settlements	0	19	16
Curtailments	0	0	9
Special termination benefits	0	2	16
Net periodic benefit cost	217	323	524
Discontinued operations	0	122	248
Net amount recognized in Statement of Consolidated Operations	217	201	276
Other postretirement benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	7	13	14
Interest cost	30	63	92
Expected return on plan assets	0	0	0
Recognized net actuarial loss	5	24	17
Amortization of prior service cost (benefit)	(8)	(24)	(37)
Settlements	0	0	0
Curtailments	0	0	(4)
Special termination benefits	0	0	0
Net periodic benefit cost	34	76	82
Discontinued operations	0	41	43
Net amount recognized in Statement of Consolidated Operations	34	35	39
Decrease in net periodic benefit cost for the recognition of the federal subsidy awarded under Medicare Part D	11	22	34
United States | Pension benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	$ 206	$ 261	$ 423