Pension and Other Postretirement Benefits - Schedule of Amounts Expected to be Recognized in Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss recognition		$ 185	$ 102	$ 42
Prior service cost (benefit) recognition		9	7	6
Pension Benefits [Member] | Scenario, Forecast [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss recognition	$ 223
Prior service cost (benefit) recognition	9
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss recognition		13	8	(3)
Prior service cost (benefit) recognition		$ (6)	$ (5)	$ (9)
Other Postretirement Benefits [Member] | Scenario, Forecast [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss recognition	15
Prior service cost (benefit) recognition	$ (6)