Earnings per share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Numerator:
Net income (loss)	$ 6,617	$ 1,549	$ 7,417	$ (5,638)
Denominator:
Weighted-average number of shares-basic (in shares)	36,202,801	35,855,217	36,144,242	35,552,900
Dilutive securities-equity awards (in shares)	467,293	592,716	280,388	0
Weighted-average number of shares-diluted (in shares)	36,670,094	36,447,933	36,424,630	35,552,900
Income (loss) per share-basic	$ 0.18	$ 0.04	$ 0.21	$ (0.16)
Income (loss) per share-diluted	$ 0.18	$ 0.04	$ 0.20	$ (0.16)
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti dilutive shares excluded from calculation (in shares)	3,000,000	1,700,000	2,900,000	3,900,000