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Summary of Significant Accounting Policies - Recent accounting pronouncements (Details) - ASU 2016-01 - USD ($)
Dec. 31, 2018
Jan. 01, 2018
Accumulated Other Comprehensive Income (Loss) [Member]    
Recent accounting pronouncements    
Reclassification on adoption of ASU 2016-01 $ (73,000)  
Retained Earnings [Member]    
Recent accounting pronouncements    
Reclassification on adoption of ASU 2016-01 $ 73,000  
Adjustment | Retained Earnings [Member]    
Recent accounting pronouncements    
Reclassification on adoption of ASU 2016-01   $ 73,000