Pension and Other Postretirement Benefits - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost			$ 108	$ 168
Net periodic benefit costs continuing and discontinued
Net periodic benefit cost			108	168
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 22	$ 41	45	81
Interest cost	58	122	116	244
Expected return on plan assets	(83)	(186)	(166)	(371)
Recognized net actuarial loss	55	102	110	204
Amortization of prior service cost (benefits)	2	4	3	8
Settlements		2		2
Special termination benefits				1
Net periodic benefit cost	54	85	108	169
Net periodic benefit costs continuing and discontinued
Discontinued operations		40		73
Net amount recognized in Statement of Consolidated Operations	54	45	108	96
Net periodic benefit cost	54	85	108	169
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	2	4	4	7
Interest cost	7	19	15	37
Recognized net actuarial loss	1	5	2	11
Amortization of prior service cost (benefits)	(2)	(7)	(4)	(13)
Net periodic benefit cost	8	21	17	42
Net periodic benefit costs continuing and discontinued
Discontinued operations		13		25
Net amount recognized in Statement of Consolidated Operations	8	8	17	17
Net periodic benefit cost	$ 8	$ 21	$ 17	$ 42