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RELATED‑PARTY TRANSACTIONS (Details)
12 Months Ended
Dec. 31, 2017
USD ($)
property
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Related Party Transaction [Line Items]      
Payable to related parties $ 20,000 $ 0 $ 0
Receivable from related parties $ 0 40,000 20,000
Related party leasing      
Related Party Transaction [Line Items]      
Lease term 5 years    
Extension option term 5 years    
Expenses with related party $ 100,000    
Number of properties adjacent to corporate office subject to leases | property 5    
Related party leasing | Property 1      
Related Party Transaction [Line Items]      
Expenses with related party $ 30,000    
Related party leasing | Property 2      
Related Party Transaction [Line Items]      
Expenses with related party 30,000    
Related party leasing | Property 3      
Related Party Transaction [Line Items]      
Expenses with related party 100,000    
Related party leasing | Property 4      
Related Party Transaction [Line Items]      
Expenses with related party 100,000    
Related party leasing | Property 5      
Related Party Transaction [Line Items]      
Expenses with related party 200,000    
Related party transportation services      
Related Party Transaction [Line Items]      
Expenses with related party 300,000 200,000 200,000
Related party equipment rental      
Related Party Transaction [Line Items]      
Expenses with related party $ 200,000 $ 200,000 $ 200,000