Debt and obligations under capital lease - Long-term debt and obligations under capital lease (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Loans from banks and other parties collateralized by tangible and intangible personal property and real estate with a maturity of 2 to 5 years (2015 - four to 13 years), with a weighted average interest rate of 4.27% (2015 - 4.28%)	$ 602,083	$ 596,428
Obligations under capital lease	19	59
Total long-term debt and obligations under capital leases	602,102	596,487
Less: Current portion	5,284	5,349
Less: Discount on secured term loan	1,515	1,894
Less: Debt issuance costs	9,535	11,701
Non-current portion of long-term debt and obligations under capital lease	$ 585,768	$ 577,543
Weighted average interest rate	4.27%	4.28%
Minimum
Debt Instrument [Line Items]
Period of debt repayment	2 years	4 years
Maximum
Debt Instrument [Line Items]
Period of debt repayment	5 years	13 years