Quarterly report pursuant to Section 13 or 15(d)

Restricted Cash (Details)

v3.8.0.1
Restricted Cash (Details) - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Mar. 31, 2017
Restricted Cash      
Restricted cash $ 7,458 $ 7,558 $ 7,557
Surety Bond [Member]      
Restricted Cash      
Reclamation bonding requirement 27,000    
Certificate of deposit [Member]      
Restricted Cash      
Restricted cash   100  
Money Market Funds [Member]      
Restricted Cash      
Restricted cash 7,458 $ 7,458  
Money Market Funds [Member] | Surety Bond [Member]      
Restricted Cash      
Restricted cash pledged as collateral $ 7,500