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Accounts receivable (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Accounts receivable, net $ 138,478 $ 113,906
Increase in unbilled accounts receivable 29,900  
Billed [Member]    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Accounts receivable, net 90,439 95,735
Unbilled [Member]    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Accounts receivable, net 48,039 18,171
Accounts Receivable [Member]    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Accounts receivable, net $ 138,478 $ 113,906