Benefit Plans - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 8	$ 12	$ 16
Interest cost	128	127	149
Expected return on plan assets	(177)	(193)	(209)
Amortization of unrecognized net loss	46	42	30
Amortization of unrecognized prior service benefit	(1)	(1)	(1)
Settlement/Curtailment	3	3	86
Net periodic benefit cost	7	(10)	71
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	1	1	1
Interest cost	3	3	4
Expected return on plan assets	(5)	(5)	(4)
Amortization of unrecognized net loss		1	1
Amortization of unrecognized prior service benefit	(3)	(10)	(18)
Settlement/Curtailment			(86)
Net periodic benefit cost	$ (4)	$ (10)	$ (102)