Condensed Consolidated Statements of Stockholders Equity - USD ($) $ in Thousands	Common Stock	Additional Paid In Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit	Total
Balance at Dec. 31, 2017	$ 1,205	$ 1,380,577	$ (750)	$ (860,614)	$ 520,418
Balance (in shares) at Dec. 31, 2017	120,497,426
Increase (Decrease) in Stockholders' Equity
Equity component of convertible notes, net of issuance costs		189,456			189,456
Exercise of common stock options	$ 4	1,387			1,391
Exercise of common stock options (in shares)	420,129
Issuance of common stock to fund the Company's 401(k) match	$ 1	4,299			4,300
Issuance of common stock to fund the Company's 401(k) match (in shares)	86,828
Compensation expense related to issuance of stock options and restricted stock awards	$ 9	12,454			12,463
Compensation expense related to issuance of stock options and restricted stock awards (in shares)	862,376
Net loss				(39,424)	(39,424)
Accumulated other comprehensive loss			(1,586)		(1,586)
Balance at Mar. 31, 2018	$ 1,219	1,588,173	(2,336)	(900,038)	687,018
Balance (in shares) at Mar. 31, 2018	121,866,759
Balance at Dec. 31, 2017	$ 1,205	1,380,577	(750)	(860,614)	520,418
Balance (in shares) at Dec. 31, 2017	120,497,426
Increase (Decrease) in Stockholders' Equity
Net loss					(75,811)
Accumulated other comprehensive loss					(1,128)
Balance at Jun. 30, 2018	$ 1,226	1,681,465	(1,878)	(936,425)	744,388
Balance (in shares) at Jun. 30, 2018	122,604,714
Balance at Mar. 31, 2018	$ 1,219	1,588,173	(2,336)	(900,038)	687,018
Balance (in shares) at Mar. 31, 2018	121,866,759
Increase (Decrease) in Stockholders' Equity
Equity component of convertible notes, net of issuance costs		70,788			70,788
Exercise of common stock options	$ 3	4,250			4,253
Exercise of common stock options (in shares)	365,566
Issuance of common stock to fund the Company's 401(k) match		3			3
Issuance of common stock to fund the Company's 401(k) match (in shares)	54
Compensation expense related to issuance of stock options and restricted stock awards	$ 1	15,592			15,593
Compensation expense related to issuance of stock options and restricted stock awards (in shares)	87,322
Purchase of employee stock purchase plan shares	$ 3	2,659			2,662
Purchase of employee stock purchase plan shares (in shares)	285,013
Net loss				(36,387)	(36,387)
Accumulated other comprehensive loss			458		458
Balance at Jun. 30, 2018	$ 1,226	1,681,465	(1,878)	(936,425)	744,388
Balance (in shares) at Jun. 30, 2018	122,604,714
Balance at Dec. 31, 2018	$ 1,232	1,716,894	(1,422)	(1,035,763)	$ 680,941
Balance (in shares) at Dec. 31, 2018	123,192,540				123,192,540
Increase (Decrease) in Stockholders' Equity
Equity component of convertible notes, net of issuance costs		268,390			$ 268,390
Shares issued to settle convertible notes	$ 22	182,413			182,435
Shares issued to settle convertible notes (in shares)	2,158,991
Settlement of convertible notes		(300,768)			(300,768)
Exercise of common stock options	$ 2	3,648			3,650
Exercise of common stock options (in shares)	235,278
Issuance of common stock to fund the Company's 401(k) match	$ 1	7,408			7,409
Issuance of common stock to fund the Company's 401(k) match (in shares)	86,532
Compensation expense related to issuance of stock options and restricted stock awards	$ 35	16,131			16,166
Compensation expense related to issuance of stock options and restricted stock awards (in shares)	3,410,481
Net loss				(83,059)	(83,059)
Accumulated other comprehensive loss			1,776		1,776
Balance at Mar. 31, 2019	$ 1,292	1,894,116	354	(1,118,822)	776,940
Balance (in shares) at Mar. 31, 2019	129,083,822
Balance at Dec. 31, 2018	$ 1,232	1,716,894	(1,422)	(1,035,763)	$ 680,941
Balance (in shares) at Dec. 31, 2018	123,192,540				123,192,540
Increase (Decrease) in Stockholders' Equity
Shares issued to settle convertible notes					$ 182,435
Shares issued to settle convertible notes (in shares)					2,158,991
Net loss					$ (121,479)
Balance at Jun. 30, 2019	$ 1,295	1,919,719	1,760	(1,157,242)	$ 765,532
Balance (in shares) at Jun. 30, 2019	129,361,048				129,361,048
Balance at Mar. 31, 2019	$ 1,292	1,894,116	354	(1,118,822)	$ 776,940
Balance (in shares) at Mar. 31, 2019	129,083,822
Increase (Decrease) in Stockholders' Equity
Equity component of convertible notes, net of issuance costs		(22)			(22)
Exercise of common stock options	$ 1	1,347			1,348
Exercise of common stock options (in shares)	78,793
Compensation expense related to issuance of stock options and restricted stock awards	$ 1	20,142			20,143
Compensation expense related to issuance of stock options and restricted stock awards (in shares)	104,845
Purchase of employee stock purchase plan shares	$ 1	4,136			4,137
Purchase of employee stock purchase plan shares (in shares)	93,588
Net loss				(38,420)	(38,420)
Accumulated other comprehensive loss			1,406		1,406
Balance at Jun. 30, 2019	$ 1,295	$ 1,919,719	$ 1,760	$ (1,157,242)	$ 765,532
Balance (in shares) at Jun. 30, 2019	129,361,048				129,361,048