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Sale of China Business - Summary of Discontinued Operations (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Disclosure of Discontinued Operations [line items]    
Net loss from discontinued operations $ (2,797) $ (339,369)
China Business [member]    
Disclosure of Discontinued Operations [line items]    
Revenue   217,511
Production costs   144,590
Depreciation and amortization   19,067
Gross profit   53,854
Exploration expenses   1,257
General and administrative expenses   20,999
Foreign exchange loss   306
Operating profit   31,292
Interest and financing costs   169
Asset retirement obligation accretion   356
Other expense   2,713
Profit from discontinued operations before income tax   28,054
Income tax expense   16,189
Profit (loss) from discontinued operations   11,865
Loss on sale of assets held for sale   351,234
Net loss from discontinued operations   $ (339,369)