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Asset Retirement Obligations (Details) - USD ($)
$ in Thousands
3 Months Ended 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Jun. 30, 2016
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Changes in carrying amount of asset retirement obligations            
Balance at the beginning of the period       $ 98,166 $ 153,155  
Accretion       7,072 6,645 $ 12,555
Revisions to estimated future reclamation cash flows       (3,802) (60,681)  
Payments       (1,126) (953)  
Balance at the end of the period $ 100,310 $ 98,166   100,310 98,166 $ 153,155
Less: current portion (1,013) (1,118)   (1,013) (1,118)  
Asset retirement obligations, net of current portion 99,297 97,048   99,297 97,048  
Concurrent reclamation excluded from asset retirement obligation 3,300 4,600   3,300 4,600  
ARO Asset Revisions       (2,300) (8,700)  
Non-cash credit to depreciation and depletion expense $ 2,800 $ 13,900 $ 37,300 2,900 53,300  
Antelope Mine            
Changes in carrying amount of asset retirement obligations            
Revisions to estimated future reclamation cash flows       2,700 22,300  
Cordero Rojo Mine            
Changes in carrying amount of asset retirement obligations            
Revisions to estimated future reclamation cash flows       2,600 34,700  
Spring Creek Mine            
Changes in carrying amount of asset retirement obligations            
Revisions to estimated future reclamation cash flows       $ 200 $ 1,900