Properties, Plants, and Equipment, Net - Additional Information (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($) kt	Dec. 31, 2017 USD ($) kt
Smelting Assets [Member]
Property, Plant and Equipment [Line Items]
Net carrying value of assets | $	$ 427	$ 248
Idle capacity of assets, units | kt	916	856
Idled Refining Assets [Member]
Property, Plant and Equipment [Line Items]
Net carrying value of assets | $	$ 132	$ 141
Idle capacity of assets, units | kt	2,305	2,305