Annual report pursuant to Section 13 and 15(d)

COMMITMENTS AND CONTINGENCIES - Additional Information (Details)

v3.19.1
COMMITMENTS AND CONTINGENCIES - Additional Information (Details)
12 Months Ended
Dec. 31, 2018
USD ($)
Railcar
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Dec. 31, 2018
SAR (ر.س)
Railcar
Dec. 31, 2017
SAR (ر.س)
Oct. 24, 2010
USD ($)
Oct. 24, 2010
SAR (ر.س)
Guarantor Obligations [Line Items]              
Undiscounted future minimum operating lease payments not recognized under current standard $ 18,131,000            
Rental expense of operating leases 4,400,000 $ 4,400,000 $ 4,200,000        
Mine closure and environmental $ 745,000 593,000 $ 622,000        
Railroad Transportation Equipment              
Guarantor Obligations [Line Items]              
Number of railcars | Railcar 346     346      
Undiscounted future minimum operating lease payments not recognized under current standard $ 17,300,000            
Term of lease commitment 9 years     9 years      
Office Space in Sugar Land, TX              
Guarantor Obligations [Line Items]              
Undiscounted future minimum operating lease payments not recognized under current standard $ 587,000            
Other Machinery and Equipment              
Guarantor Obligations [Line Items]              
Undiscounted future minimum operating lease payments not recognized under current standard 207,000            
Saudi Industrial Development Fund Limited Guarantee              
Guarantor Obligations [Line Items]              
Loan guarantee           41.00% 41.00%
Principal amount of loan guaranteed           $ 88,000,000.0 ر.س 330,000,000.0
Amount of maximum exposure $ 81,300,000 $ 81,300,000   ر.س 305,000,000 ر.س 305,000,000 $ 36,100,000 ر.س 135,300,000