Pension and Other Postretirement Benefits - Schedule of Amounts Expected to be Recognized in Net Periodic Benefit Cost (Detail) $ in Millions	Dec. 31, 2017 USD ($)
Pension benefits
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss recognition	$ 168
Prior service cost (benefit) recognition	3
Other postretirement benefits
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss recognition	9
Prior service cost (benefit) recognition	$ (8)