Income taxes - Tax impact of shortfalls (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Deferred income tax stock based compensation tax deficiency	$ 3,334	$ 270	$ 575
Restricted stock awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Deferred income tax stock based compensation tax deficiency	3,334	112	425
Restricted stock option awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Deferred income tax stock based compensation tax deficiency	$ 0	$ 158	$ 150