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Revenue Recognition - Contract Liabilities (Details) - USD ($)
6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Dec. 31, 2018
Deferred revenue        
Advance payments from customers     $ 75,830 $ 180,425
Shipments in-transit     121,599 9,582
Extended warranty agreements     3,658,078 3,415,782
Total $ 3,605,789 $ 3,621,256 $ 3,855,507 $ 3,605,789
Changes in contract liabilities        
Contract liabilities at beginning of the year 3,605,789 3,621,256    
Increases due to cash received from customers 1,646,638 1,314,024    
Decreases due to recognition of revenue (1,396,920) (1,008,979)    
Contract liabilities at end of the year $ 3,855,507 $ 3,926,301