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Property, Plant and Mine Development (Tables)
 6 Months Ended
Jun. 30, 2017
Property, Plant and Equipment [Abstract]  
Schedule of Property, Plant and Mine Development
At June 30, 2017 and December 31, 2016, property, plant and mine development (in thousands) consisted of the following:
 
June 30, 
 2017
 
December 31,  
 2016
Mining property and mine development
$
512,272

 
$
414,434

Asset retirement cost
8,062

 
8,062

Land
35,052

 
35,052

Land improvements
43,159

 
42,738

Buildings
52,229

 
52,178

Machinery and equipment
497,115

 
450,881

Furniture and fixtures
2,635

 
2,566

Construction-in-progress
76,191

 
43,790

 
1,226,715

 
1,049,701

Accumulated depletion, depreciation and amortization
(306,875
)
 
(266,388
)
Total property, plant and mine development, net
$
919,840

 
$
783,313