Benefit Plans - Amounts Recognized in Accumulated Other Comprehensive Income (Loss), Not Yet Recognized in Net Periodic (Benefit) Cost (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Prior service credit		$ (2)
Net actuarial loss	$ 1,144	1,065
Net amount recognized	1,144	1,063
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Prior service credit	(1)	(1)
Net actuarial loss	(4)	(3)
Net amount recognized	$ (5)	$ (4)