CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY (DEFICIT) - USD ($) shares in Thousands, $ in Thousands	Common Stock	Additional Paid-In Capital	Accumulated Deficit	Total
Balance at Dec. 31, 2019	$ 1,501	$ 1,209,846	$ (1,287,468)	$ (76,121)
Balance (in shares) at Dec. 31, 2019	150,136
Increase (Decrease) in Shareholders' Equity (Deficit)
Net loss			(29,088)	(29,088)
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan	$ 1	239		240
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan (in shares)	86
Issuance of common stock, net of issuance costs	$ 245	97,499		97,744
Issuance of common stock, net of issuance costs (in shares)	24,524
Stock option and restricted stock compensation expense		3,122		3,122
Restricted stock units vested (in shares)	2
Restricted stock award, net of forfeitures	$ (4)	4
Restricted stock award, net of forfeitures (in shares)	(487)
Balance at Mar. 31, 2020	$ 1,743	1,310,710	(1,316,556)	(4,103)
Balance (in shares) at Mar. 31, 2020	174,261
Balance at Dec. 31, 2019	$ 1,501	1,209,846	(1,287,468)	(76,121)
Balance (in shares) at Dec. 31, 2019	150,136
Increase (Decrease) in Shareholders' Equity (Deficit)
Net loss				(75,760)
Balance at Sep. 30, 2020	$ 1,746	1,318,591	(1,363,228)	(42,891)
Balance (in shares) at Sep. 30, 2020	174,585
Balance at Mar. 31, 2020	$ 1,743	1,310,710	(1,316,556)	(4,103)
Balance (in shares) at Mar. 31, 2020	174,261
Increase (Decrease) in Shareholders' Equity (Deficit)
Net loss			(24,298)	(24,298)
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan	$ 1	424		425
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan (in shares)	122
Stock option and restricted stock compensation expense		3,409		3,409
Adjustment of issuance costs		(1)		(1)
Restricted stock units vested	$ 1	(1)
Restricted stock units vested (in shares)	157
Directors' deferred share unit compensation		45		45
Balance at Jun. 30, 2020	$ 1,745	1,314,586	(1,340,854)	(24,523)
Balance (in shares) at Jun. 30, 2020	174,540
Increase (Decrease) in Shareholders' Equity (Deficit)
Net loss			(22,374)	(22,374)
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan	$ 1	127		128
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan (in shares)	45
Stock option and restricted stock compensation expense		3,729		3,729
Directors' deferred share unit compensation		149		149
Balance at Sep. 30, 2020	$ 1,746	1,318,591	(1,363,228)	(42,891)
Balance (in shares) at Sep. 30, 2020	174,585
Increase (Decrease) in Shareholders' Equity (Deficit)
Net loss			31,388	31,388
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan	$ 2	676		678
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan (in shares)	205
Issuance of common stock, net of issuance costs	$ 200	96,328		96,528
Issuance of common stock, net of issuance costs (in shares)	19,972
Stock option and restricted stock compensation expense		3,718		3,718
Restricted stock units vested	$ 2	(2)
Restricted stock units vested (in shares)	236
Directors' deferred share unit compensation		149		149
Balance at Dec. 31, 2020	$ 1,950	1,419,460	(1,331,840)	$ 89,570
Balance (in shares) at Dec. 31, 2020	194,998			194,998
Increase (Decrease) in Shareholders' Equity (Deficit)
Net loss			(34,051)	$ (34,051)
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan	$ 4	1,282		1,286
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan (in shares)	397
Issuance of common stock, net of issuance costs	$ 45	33,447		33,492
Issuance of common stock, net of issuance costs (in shares)	4,544
Stock option and restricted stock compensation expense		3,674		3,674
Restricted stock units vested (in shares)	2
Directors' deferred share unit compensation		149		149
Balance at Mar. 31, 2021	$ 1,999	1,458,012	(1,365,891)	94,120
Balance (in shares) at Mar. 31, 2021	199,941
Balance at Dec. 31, 2020	$ 1,950	1,419,460	(1,331,840)	$ 89,570
Balance (in shares) at Dec. 31, 2020	194,998			194,998
Increase (Decrease) in Shareholders' Equity (Deficit)
Net loss				$ (102,131)
Balance at Sep. 30, 2021	$ 2,024	1,509,040	(1,433,971)	$ 77,093
Balance (in shares) at Sep. 30, 2021	202,443			202,443
Balance at Mar. 31, 2021	$ 1,999	1,458,012	(1,365,891)	$ 94,120
Balance (in shares) at Mar. 31, 2021	199,941
Increase (Decrease) in Shareholders' Equity (Deficit)
Net loss			(30,741)	(30,741)
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan	$ 1	377		378
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan (in shares)	75
Stock option and restricted stock compensation expense		3,598		3,598
Conversion of debt	$ 3	997		1,000
Conversion of debt (in shares)	239
Common stock issuance costs		(34)		(34)
Directors' deferred share unit compensation		144		144
Balance at Jun. 30, 2021	$ 2,003	1,463,094	(1,396,632)	68,465
Balance (in shares) at Jun. 30, 2021	200,255
Increase (Decrease) in Shareholders' Equity (Deficit)
Net loss			(37,339)	(37,339)
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan	$ 1	367		368
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan (in shares)	95
Issuance of common stock, net of issuance costs	$ 21	12,336		12,357
Issuance of common stock, net of issuance costs (in shares)	2,150
Issuance of pre-funded warrant, net of issuance costs		29,765		29,765
Stock option and restricted stock compensation expense		3,298		3,298
Directors' deferred share units converted		179		179
Restricted stock award, net of forfeitures	$ (1)	1
Restricted stock award, net of forfeitures (in shares)	(57)
Balance at Sep. 30, 2021	$ 2,024	$ 1,509,040	$ (1,433,971)	$ 77,093
Balance (in shares) at Sep. 30, 2021	202,443			202,443