Earnings per share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2016		Jun. 30, 2015		Jun. 30, 2016	Jun. 30, 2015
Numerator [Abstract]
Net income (loss)	$ (10,947)		$ 14,100		$ (6,956)	$ (7,419)
Interest expense, net of tax	0		809		0	0
Amortization of debt issuance costs, net of tax	0		219		0	0
Net income (loss), adjusted	$ (10,947)		$ 15,128		$ (6,956)	$ (7,419)
Denominator [Abstract]
Weighted-average number of shares-basic (in shares)	40,202,821		38,480,754		39,872,738	38,216,524
Dilutive securities-equity awards (in shares)	0		1,209,134		0	0
Dilutive securities-convertible debt	0		7,720,525		0	0
Weighted-average number of shares-diluted (in shares)	40,202,821		47,410,413		39,872,738	38,216,524
Net income (loss) per share - basic	$ (0.27)		$ 0.37		$ (0.17)	$ (0.19)
Adjusted net (loss) income per share-diluted	$ (0.27)	[1]	$ 0.32	[1]	$ (0.17)	$ (0.19)
Stock Options [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive shares excluded from calculation (in shares)	3,500,000				3,500,000	4,700,000
Convertible debt [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Interest rate percentage	2.875%				2.875%
Antidilutive shares excluded from calculation (in shares)	7,700,000				7,700,000	7,700,000

[1] See Note 8 "Earnings per share" for details on calculation.