Earnings per share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Numerator:
Net income	$ 16,104	$ 6,617	$ 7,632	$ (6,829)	$ 28,657	$ 1,549	$ 14,210	$ (21,397)	$ 23,524	$ 23,019	$ 51,698
Denominator:
Weighted-average number of shares-basic (in shares)									36,080,495	35,658,907	31,782,286
Dilutive securities-equity awards (in shares)									340,167	547,145	757,214
Weighted-average number of shares-diluted (in shares)									36,420,662	36,206,052	32,539,500
Earnings per share-basic (in dollars per share)	$ 0.45	$ 0.18	$ 0.21	$ (0.19)	$ 0.80	$ 0.04	$ 0.40	$ (0.61)	$ 0.65	$ 0.65	$ 1.63
Earnings per share-diluted (in dollars per share)	$ 0.44	$ 0.18	$ 0.21	$ (0.19)	$ 0.78	$ 0.04	$ 0.39	$ (0.61)	$ 0.65	$ 0.64	$ 1.59
Stock Options [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive shares excluded from calculation (in shares)									2,900,000	746,000	1,400,000