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Impairment of Investment in Equity Investees (Parenthetical) (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2015
Jun. 30, 2015
Dec. 31, 2015
Dec. 31, 2014
Impairment Of Assets [Line Items]        
Total impairment of investment in equity investees     $ 109.5 $ 7.4
Hummingbird        
Impairment Of Assets [Line Items]        
Total impairment of investment in equity investees   $ 7.5 7.5 [1]  
Far South East Project [Member]        
Impairment Of Assets [Line Items]        
Total impairment of investment in equity investees $ 101.4   $ 101.4 [2]  
Equity method investment ownership percentage 40.00%   40.00% 40.00%
Far South East Project [Member] | Lepanto Consolidated Mining Company        
Impairment Of Assets [Line Items]        
Equity method investment ownership percentage 60.00%   60.00%  
[1] Following the identification of impairment indicators at June 30, 2015, the carrying amount of the investment in Hummingbird was impaired to its fair value, being its quoted market price, which resulted in an impairment of $7.5 million.
[2] Following the identification of impairment indicators at December 31, 2015, the carrying value of FSE was impaired to its fair value which resulted in an impairment of $101.4 million. The fair value was indirectly derived from the market value of Lepanto Consolidated Mining Company, being the 60% shareholder of FSE.