Benefit Plans - Amounts Recognized in Accumulated Other Comprehensive Income (Loss) Not Yet Recognized in Net Periodic (Benefit) Cost (Detail) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Prior service credit	$ (3)	$ (5)
Net actuarial loss	1,097	1,106
Net amount recognized	1,094	1,101
Projected benefit obligation	3,103	3,129
Accumulated benefit obligation	3,089	3,114
Fair value of plan assets	2,391	2,391
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Prior service credit	(6)	(9)
Net actuarial loss	(2)	8
Net amount recognized	(8)	(1)
Accumulated benefit obligation	$ 24	$ 34