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Income taxes - Components of unrecognized tax positions (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Income Tax Disclosure [Abstract]        
Highly certain tax positions $ 3,700 $ 1,900 $ 1,100  
Other unrecognized tax benefits 100 100 100  
Gross unrecognized tax benefits 3,800 2,000 1,200 $ 17,600
Accrued interest and penalties related to unrecognized tax benefits $ 0 $ 0 $ 100