Income taxes (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Income taxes
Windfall tax benefits from stock option exercises			$ 2,459
Income tax (benefit) expense	258	1,101	(382)	2,468
Effective tax rate (as a percent)			35.60%	27.60%
Deferred tax assets	2,394		2,394
Uncertain tax positions	392		392		392
Uncertain tax positions, reduction to deferred tax assets	$ 106		$ 106		$ 106