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, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss recognition		$ 171	$ 198	$ 185
Amortization of prior service cost (benefit)(3)		4	8	9
Pension Benefits [Member] | Scenario Forecast [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss recognition	$ 202
Amortization of prior service cost (benefit)(3)	1
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss recognition		$ 10	$ 13	13
Amortization of prior service cost (benefit)(3)				$ (6)
Other Postretirement Benefits [Member] | Scenario Forecast [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss recognition	17
Amortization of prior service cost (benefit)(3)	$ (14)