Income taxes - Schedule of Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefits, beginning balance	$ 1,583	$ 1,574	$ 5,236
Additions for tax positions of prior years	907	0	1,168
Additions for tax positions of current year	0	9	77
Reclass to other tax accounts	0	0	(4,907)
Unrecognized tax benefits, ending balance	$ 2,490	$ 1,583	$ 1,574