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Long-term debt - Capital Lease Obligations to Related Party (Details) - USD ($)
3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Dec. 31, 2018
Dec. 31, 2013
Dec. 31, 2017
Essex sale-leaseback transactions | Transaction One              
Long-term debt              
Imputed interest rate on lease (as a percent)           14.90%  
Financing and capital leases, maturing through May 2024              
Long-term debt              
Interest expense $ 58,000 $ 91,000 $ 122,000 $ 189,000      
Financing and capital leases, maturing through May 2024 | Equipment lease              
Long-term debt              
Newly acquired asset under equipment lease $ 400,000   $ 400,000   $ 100,000    
Interest rate on lease (as a percent) 6.50%       5.30%    
Financing and capital leases, maturing through May 2024 | Essex sale-leaseback transactions | Transaction Two              
Long-term debt              
Newly acquired assets under sale-leaseback transaction             $ 3,200,000