Earnings per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Numerator, in thousands:
Net income used for calculation of basic EPS	$ 17,368	$ 10,251	$ 27,665	$ 15,076
Interest expense on convertible debt	58	196	147	375
Changes in fair value of derivative liabilities	(23)	(123)	(76)	(224)
Loss on extinguishment of debt	103		204	382
Loss on extinguishment of outstanding debt, as if converted	(258)	(849)	(321)	(1,183)
Total adjustments	(120)	(776)	(46)	(650)
Net income used for calculation of diluted EPS	$ 17,248	$ 9,475	$ 27,619	$ 14,426
Denominator:
Weighted average shares outstanding, basic	50,530,968	49,427,825	50,345,830	49,333,962
Effect of dilutive potential common shares:
Shares underlying Convertible Senior Secured Notes	421,708	1,240,814	551,235	1,301,885
Shares issuable to settle interest make-whole derivatives	4,631	52,563	7,013	71,537
Stock options and stock appreciation rights	2,266,407	1,024,140	2,122,245	777,302
Total potential dilutive common shares	2,692,746	2,317,517	2,680,493	2,150,724
Weighted average shares outstanding, diluted	53,223,714	51,745,342	53,026,323	51,484,686
Net income per share, basic	$ 0.34	$ 0.21	$ 0.55	$ 0.31
Net income per share, diluted	$ 0.32	$ 0.18	$ 0.52	$ 0.28
Stock options, stock appreciation rights, and ESPP awards
Income per share
Common stock equivalents excluded in the calculation of diluted income per share	122,666	1,124,100	206,448	1,159,100