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Summary of significant accounting policies - Transactions with Affiliate (Details) - Purification Cellutions LLC
Dec. 31, 2017
Dec. 31, 2016
May 15, 2016
Dec. 31, 2015
Related Party Transaction [Line Items]        
Noncontrolling interest ownership percentage 30.00% 30.00% 30.00% 30.00%
Percentage of earnings included in calculation of provision for income taxes 70.00%