Employee Benefits - Pre-tax AOCI Not Yet Recognized as Net Postretirement Benefit Cost (Detail) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Total unrecognized AOCI	$ 550	$ 593
Other Postretirement Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Net unrecognized losses (gains)	(9)	(5)
Net unrecognized prior service credit	(2)	(3)
Total unrecognized AOCI	$ (11)	$ (8)