CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY - USD ($) shares in Thousands, $ in Thousands	Common Stock	Additional Paid-In Capital	Accumulated Deficit	Total
Balance at Dec. 31, 2020	$ 1,950	$ 1,419,460	$ (1,331,840)	$ 89,570
Balance (in shares) at Dec. 31, 2020	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
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 convertible senior notes	$ 3	997		1,000
Conversion of convertible senior notes (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 warrants, net of issuance costs		29,765		29,765
Stock option and restricted stock compensation expense		3,298		3,298
Restricted stock award forfeitures	$ (1)	1
Restricted stock award forfeitures (in shares)	(57)
Directors' deferred share unit compensation		179		179
Balance at Sep. 30, 2021	$ 2,024	1,509,040	(1,433,971)	77,093
Balance (in shares) at Sep. 30, 2021	202,443
Increase (Decrease) in Shareholders' Equity (Deficit)
Net loss			(37,172)	(37,172)
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan	$ 4	1,733		1,737
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan (in shares)	431
Issuance of common stock, net of issuance costs	$ 176	108,039		108,215
Issuance of common stock, net of issuance costs (in shares)	17,487
Issuance of pre-funded warrants, net of issuance costs		169,280		169,280
Stock option and restricted stock compensation expense		6,224		6,224
Directors' deferred share unit compensation		209		209
Balance at Dec. 31, 2021	$ 2,204	1,794,525	(1,471,143)	$ 325,586
Balance (in shares) at Dec. 31, 2021	220,361			220,361
Increase (Decrease) in Shareholders' Equity (Deficit)
Net loss			(24,145)	$ (24,145)
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan	$ 1	619		620
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan (in shares)	173
Stock option and restricted stock compensation expense		4,196		4,196
Restricted stock units vested (in shares)	2
Directors' deferred share unit compensation		211		211
Balance at Mar. 31, 2022	$ 2,205	$ 1,799,551	$ (1,495,288)	$ 306,468
Balance (in shares) at Mar. 31, 2022	220,536			220,536