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Significant Accounting Policies - Additional Information (Detail)
 12 Months Ended
Dec. 31, 2019
Disclosure Of Significant Accounting Policies [Line Items]  
Percentage of plant capacity 70.00%
Percentage of bid amount to be included in inventory 51.00%
De Beers [Member]  
Disclosure Of Significant Accounting Policies [Line Items]  
Percentage of bid price as revenue 49.00%