Income taxes (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Income taxes
Windfall tax benefits from stock option exercises	$ 4,039
Income tax (benefit) expense	(467)	658
Effective tax rate (as a percent)	32.10%	26.70%
Deferred tax assets	2,399
Uncertain tax positions	392		392
Uncertain tax positions, reduction to deferred tax assets	$ 106		$ 106