Consolidated Statements of Changes in Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Treasury Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Accumulated Deficit [Member]
Balances at Dec. 31, 2018	$ 862,971	$ 5	$ (211)	$ 1,827,021	$ (515)	$ (963,329)
Balances, Shares at Dec. 31, 2018		46,888,263	3,033
Issuance of common stock from exercise of stock options, Amount	14,072			14,072
Issuance of common stock from exercise of stock options, Shares		287,659
Issuance of common stock under the employee stock purchase plan	1,799			1,799
Issuance of common stock under the employee stock purchase plan, Shares		16,398
Purchase of treasury stock, Amount	(189)		$ (189)
Stock-based compensation expense	43,622			43,622
Public offering of common stock, net of offering costs, Amount	560,948			560,948
Public offering of common stock, net of offering costs, Shares		3,833,334
Unrealized gain (loss) on available-for-sale securities	409				409
Vesting of restricted stock units, net of employee tax obligations	(692)			(692)
Vesting of restricted stock units, net of employee tax obligations, Shares		8,518
Net loss	(163,406)					(163,406)
Balances at Mar. 31, 2019	1,319,534	$ 5	$ (400)	2,446,770	(106)	(1,126,735)
Balances, Shares at Mar. 31, 2019		51,034,172	3,033
Balances at Dec. 31, 2019	944,655	$ 5	$ (400)	2,587,322	1,295	(1,643,567)
Balances, Shares at Dec. 31, 2019		51,877,194	3,033
Issuance of common stock from exercise of stock options, Amount	367			367
Issuance of common stock from exercise of stock options, Shares		7,196
Issuance of common stock under the employee stock purchase plan	2,793			2,793
Issuance of common stock under the employee stock purchase plan, Shares		33,429
Stock-based compensation expense	30,218			30,218
Unrealized gain (loss) on available-for-sale securities	(2,087)				(2,087)
Net loss	(126,740)					(126,740)
Balances at Mar. 31, 2020	$ 849,206	$ 5	$ (400)	$ 2,620,700	$ (792)	$ (1,770,307)
Balances, Shares at Mar. 31, 2020		51,917,819	3,033