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Summary of Significant Accounting Policies - Summary of Expected Useful Lives (Detail)
 12 Months Ended
Dec. 31, 2018
Bottom of range [member] | Buildings and equipment (not used in production) [member]  
Disclosure of detailed information about property, plant and equipment [line items]  
Expected useful lives 2 years
Bottom of range [member] | Plant and equipment (smelting operations) [member]  
Disclosure of detailed information about property, plant and equipment [line items]  
Expected useful lives 3 years
Top of range [member] | Buildings and equipment (not used in production) [member]  
Disclosure of detailed information about property, plant and equipment [line items]  
Expected useful lives 50 years
Top of range [member] | Plant and equipment (smelting operations) [member]  
Disclosure of detailed information about property, plant and equipment [line items]  
Expected useful lives 30 years