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Income taxes (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2015
Sep. 30, 2014
Sep. 30, 2015
Sep. 30, 2014
Dec. 31, 2014
Income Tax Disclosure [Abstract]          
Income tax expense (benefit) $ 8,474 $ (2,947) $ 22,975 $ 2,574  
Effective tax rate (percent) 31.10% (25.20%) 24.60% 30.60%  
Gross unrecognized tax benefits $ 25,200   $ 25,200   $ 16,600
Unrecognized tax benefits that would be recorded as reduction in income tax provision, if recognized $ 23,600   $ 23,600