Earnings per share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Numerator:
Net income (loss)	$ 7,632	$ 14,210	$ 800	$ (7,187)
Denominator:
Weighted-average number of shares-basic (in shares)	36,182,826	35,619,514	36,114,400	35,400,906
Dilutive securities-equity awards (in shares)	373,871	1,047,938	186,935	0
Weighted-average number of shares-diluted (in shares)	36,556,697	36,667,452	36,301,335	35,400,906
Income (loss) per share-basic	$ 0.21	$ 0.40	$ 0.02	$ (0.2)
Income (loss) per share-diluted	$ 0.21	$ 0.39	$ 0.02	$ (0.2)
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti dilutive shares excluded from calculation (in shares)	3,000,000	719,000	3,000,000	4,000,000