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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Allowance for Doubtful Accounts (Details) - USD ($)
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Accounting Policies [Abstract]    
Allowance for doubtful accounts $ 2,357,130 $ 2,135,877
Provision for doubtful accounts 268,990 543,621
Allowance for doubtful accounts receivable, uncollectible $ 48,000 $ 3,160,000