Inventories - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Inventory Disclosure [Abstract]
Fixed Production Overhead To Cost Of Sales	$ 2,735	$ 2,757	$ 3,232
- Inventory provision (note 6)	$ 2,529	$ 1,231	$ 6,377

Amount of loss from reductions in inventory due to subsequent measurement adjustments, including, but not limited to, physical deterioration, obsolescence, or changes in price levels.

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