Earnings per share (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Numerator [Abstract]
Net Income		$ 50,144	$ 4,616	$ 45,236	$ 15,101
Interest expense, net of tax		0	835	0	1,742
Amortization of debt issuance costs, net of tax		0	195	0	391
Net income, adjusted		$ 50,144	$ 5,646	$ 45,236	$ 17,234
Denominator [Abstract]
Weighted-average number of shares-basic (in shares)		49,896,124	41,013,764	49,738,980	40,871,540
Dilutive securities-equity awards (in shares)		1,266,785	968,354	1,300,215	931,263
Dilutive securities-convertible debt (in shares)		0	8,096,476	0	8,096,488
Weighted-average number of shares-diluted (in shares)		51,162,909	50,078,594	51,039,195	49,899,291
Net income per share-basic (in dollars per share)		$ 1.00	$ 0.11	$ 0.91	$ 0.37
Net income per share-diluted (in dollars per share)	[1]	$ 0.98	$ 0.11	$ 0.89	$ 0.35
Stock Options [Member]
Antidilutive shares excluded from computation of earnings per share [Abstract]
Antidilutive shares excluded from calculation (in shares)			400,000		400,000
Convertible debt [Member]
Antidilutive shares excluded from computation of earnings per share [Abstract]
Interest rate percentage			2.875%		2.875%

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