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Income taxes (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Income taxes
Windfall tax benefits from stock option exercises			$ 3,758
Income tax (benefit) expense	(173)	709	(640)	1,367
Effective tax rate (as a percent)			34.50%	28.60%
Deferred tax assets	2,399		2,399
Uncertain tax positions	392		392		392
Uncertain tax positions, reduction to deferred tax assets	$ 106		$ 106		$ 106

- Definition

A ratio calculated by dividing the reported amount of income tax expense attributable to continuing operations for the period by GAAP-basis pretax income from continuing operations.

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- Definition

The sum of the current income tax expense or benefit and the deferred income tax expense or benefit pertaining to continuing operations.

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