Condensed Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Loss	Accumulated Deficit
Beginning balance (in shares) at Dec. 31, 2017		48,826,153
Beginning balance at Dec. 31, 2017	$ 375,503	$ 49	$ 1,174,904	$ (1,389)	$ (798,061)
Issuance of common stock for follow-on offering (in shares)		8,152,986
Issuance of common stock for follow-on offering	516,206	$ 8	516,198
Common stock issued under stock incentive plan and ESPP (in shares)		562,474
Common stock issued under stock incentive plan and ESPP	12,332	$ 1	12,331
Stock-based compensation expense	14,522		14,522
Other comprehensive income (loss)	(1,254)			(1,254)
Net loss	(90,825)				(90,825)
Other	(346)		(346)
Ending balance (in shares) at Mar. 31, 2018		57,541,613
Ending balance at Mar. 31, 2018	865,594	$ 58	1,717,609	(2,643)	(849,430)
Beginning balance (in shares) at Dec. 31, 2017		48,826,153
Beginning balance at Dec. 31, 2017	375,503	$ 49	1,174,904	(1,389)	(798,061)
Net loss	(254,234)
Ending balance (in shares) at Sep. 30, 2018		58,167,932
Ending balance at Sep. 30, 2018	760,213	$ 58	1,775,113	(2,119)	(1,012,839)
Beginning balance (in shares) at Mar. 31, 2018		57,541,613
Beginning balance at Mar. 31, 2018	865,594	$ 58	1,717,609	(2,643)	(849,430)
Common stock issued under stock incentive plan and ESPP (in shares)		391,423
Common stock issued under stock incentive plan and ESPP	9,638		9,638
Stock-based compensation expense	16,455		16,455
Other comprehensive income (loss)	245			245
Net loss	(68,745)				(68,745)
Other	(45)		(45)
Ending balance (in shares) at Jun. 30, 2018		57,933,036
Ending balance at Jun. 30, 2018	823,142	$ 58	1,743,657	(2,398)	(918,175)
Common stock issued under stock incentive plan and ESPP (in shares)		234,896
Common stock issued under stock incentive plan and ESPP	7,263		7,263
Stock-based compensation expense	24,193		24,193
Other comprehensive income (loss)	279			279
Net loss	(94,664)				(94,664)
Ending balance (in shares) at Sep. 30, 2018		58,167,932
Ending balance at Sep. 30, 2018	760,213	$ 58	1,775,113	(2,119)	(1,012,839)
Beginning balance (in shares) at Dec. 31, 2018		58,218,653
Beginning balance at Dec. 31, 2018	687,537	$ 58	1,794,283	(2,171)	(1,104,633)
Common stock issued under stock incentive plan and ESPP (in shares)		441,168
Common stock issued under stock incentive plan and ESPP	6,003	$ 1	6,002
Stock-based compensation expense	18,108		18,108
Other comprehensive income (loss)	1,687			1,687
Net loss	(93,078)				(93,078)
Ending balance (in shares) at Mar. 31, 2019		58,659,821
Ending balance at Mar. 31, 2019	620,257	$ 59	1,818,393	(484)	(1,197,711)
Beginning balance (in shares) at Dec. 31, 2018		58,218,653
Beginning balance at Dec. 31, 2018	$ 687,537	$ 58	1,794,283	(2,171)	(1,104,633)
Common stock issued under stock incentive plan and ESPP (in shares)	253,323
Net loss	$ (309,122)
Ending balance (in shares) at Sep. 30, 2019		58,877,691
Ending balance at Sep. 30, 2019	448,291	$ 59	1,861,523	464	(1,413,755)
Beginning balance (in shares) at Mar. 31, 2019		58,659,821
Beginning balance at Mar. 31, 2019	620,257	$ 59	1,818,393	(484)	(1,197,711)
Common stock issued under stock incentive plan and ESPP (in shares)		89,365
Common stock issued under stock incentive plan and ESPP	2,770		2,770
Stock-based compensation expense	18,547		18,547
Other comprehensive income (loss)	974			974
Net loss	(109,871)				(109,871)
Ending balance (in shares) at Jun. 30, 2019		58,749,186
Ending balance at Jun. 30, 2019	532,677	$ 59	1,839,710	490	(1,307,582)
Common stock issued under stock incentive plan and ESPP (in shares)		128,505
Common stock issued under stock incentive plan and ESPP	3,225		3,225
Stock-based compensation expense	18,588		18,588
Other comprehensive income (loss)	(26)			(26)
Net loss	(106,173)				(106,173)
Ending balance (in shares) at Sep. 30, 2019		58,877,691
Ending balance at Sep. 30, 2019	$ 448,291	$ 59	$ 1,861,523	$ 464	$ (1,413,755)