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Weichai Transactions - Weichai Warrant (Details) - USD ($)
3 Months Ended
May 03, 2019
Apr. 23, 2019
Apr. 01, 2019
Oct. 01, 2018
Sep. 30, 2018
Dec. 31, 2018
Dec. 31, 2016
Share Purchase Agreement, Weichai Transaction              
Class of Warrant or Right [Line Items]              
EBITDA, minimum             $ 22,000,000
Net book value per share, minimum (in dollars per share)             $ 8
Aggregate amount             $ 15,000,000
Share Purchase Agreement, Weichai Transaction | Subsequent Event              
Class of Warrant or Right [Line Items]              
Volume weighted average share price percentage         50.00% 85.00%  
Trading period           20 days  
Exercisable (term)           90 days  
Share Purchase Agreement, Second Amended And Restated Warrant | Subsequent Event              
Class of Warrant or Right [Line Items]              
Volume weighted average share price percentage       50.00% 50.00%    
Trading period       20 days 20 days    
Exercisable (term)         90 days    
Share Purchase Agreement, Second Amended And Restated Warrant | Subsequent Event | Maximum              
Class of Warrant or Right [Line Items]              
Reduction of exercise price       $ 15,000,000      
Series B | Share Purchase Agreement, Second Amended And Restated Warrant | Subsequent Event              
Class of Warrant or Right [Line Items]              
Percent ownership of outstanding common stock 51.50%   51.00%        
Exercise proceeds for the warrants   $ 1,600,000          
Exercise of private placement warrants (in shares)   4,049,759