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Gain on Silver Fixed Price Swap - Additional Information (Detail) (USD $)
 3 Months Ended 1 Months Ended
Dec. 31, 2013
Fixed Price Swap Hedge [Member]
Contract
Sep. 30, 2013
Fixed Price Swap Hedge [Member]
Contract
Oct. 31, 2013
Fixed Price Swap First Contract [Member]
oz
Nov. 30, 2013
Fixed Price Swap Second Contract [Member]
oz
Derivative Instruments and Hedging Activities Disclosures [Line Items]        
Number of contracts entered 0 2    
Weight of troy ounce for first contract     250,000 250,000
Weight of troy ounce for first contract price per share     24.40 23.10
Weight of troy ounce difference multiplied description   If the average silver price was less than the fixed price, we would receive the difference multiplied by the 250,000 troy ounces. If the average silver price was greater than the fixed price, we would owe the difference multiplied by the 250,000 troy ounces.    
Combined gain from contract $ 1,206,350