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Asset Retirement Obligations (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Dec. 31, 2016
Changes in carrying amount of asset retirement obligations          
Balance at the beginning of the period     $ 98,166 $ 153,155  
Accretion expense $ 1,865 $ 1,065 5,532 5,641  
Revisions to estimated future reclamation cash flows     9,350 (53,769)  
Payments     (719) (1,048)  
Balance at the end of the period 112,329 103,979 112,329 103,979  
Less: current portion (1,118) (1,400) (1,118) (1,400)  
Asset retirement obligation, net of current portion $ 111,211 102,579 111,211 102,579 $ 97,048
ARO Asset Revisions       (17,500)  
Non-cash credit to depreciation and depletion expense   $ 36,300   36,300  
Antelope Mine          
Changes in carrying amount of asset retirement obligations          
Revisions to estimated future reclamation cash flows     2,600 24,900  
Cordero Rojo Mine          
Changes in carrying amount of asset retirement obligations          
Revisions to estimated future reclamation cash flows     10,300 20,800  
Spring Creek Mine          
Changes in carrying amount of asset retirement obligations          
Revisions to estimated future reclamation cash flows     $ 1,700 $ 8,100