Interest expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Interest Expense [Abstract]
Interest expense on long-term debt and obligations under capital lease	$ 29,425	$ 27,090	$ 27,380
Withholding tax provision classified as interest	0	0	(1,476)
Interest on legal settlements	(28)	(979)	3,294
Amortization of debt issuance costs and discount on secured term loan	3,682	3,044	2,903
Interest capitalized	(624)	0	0
Total interest expense	$ 32,455	$ 29,155	$ 32,101