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Income taxes - Components of unrecognized tax positions (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Income Tax Disclosure [Abstract]        
Highly certain tax positions $ 6,300 $ 3,700 $ 1,900  
Other unrecognized tax benefits 100 100 100  
Gross unrecognized tax benefits 6,400 3,800 2,000 $ 1,200
Accrued interest and penalties related to unrecognized tax benefits $ 0 $ 0 $ 0