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Commitments and contingencies - Additional Information (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Contingencies    
Employment Agreement Base Annual Salaries $ 5.5  
Cayman Water Retail Operations, Percentage Of Gross Profit 53.00% 54.00%
Cayman Water Retail Operations, Percentage Of Revenue 39.00% 39.00%
Long-term Purchase Commitment, Amount $ 5.5  
WSC [Member]    
Contingencies    
Accounts Receivable, Net $ 18.4 $ 17.6