Benefit Plans - Amounts Recognized in Accumulated Other Comprehensive Income (Loss) Not Yet Recognized in Net Periodic (Benefit) Cost (Detail) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Prior service credit	$ (5)	$ (5)
Net actuarial loss	1,106	1,090
Net amount recognized	1,101	1,085
Projected benefit obligation	3,129	3,336
Accumulated benefit obligation	3,114	3,262
Fair value of plan assets	2,391	2,713
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Prior service credit	(9)	(19)
Net actuarial loss	8	18
Net amount recognized	(1)	(1)
Accumulated benefit obligation	$ 34	$ 42