Other Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Other Assets [Line Items]
Other	$ 1,034	$ 5,434
Other Assets, Total	5,194	8,961
Bauxite raw materials
Other Assets [Line Items]
Inventories - bauxite raw materials	$ 4,160	$ 3,527