CONSOLIDATED STATEMENTS OF SHAREHOLDERS' (DEFICIT) EQUITY - USD ($) shares in Thousands, $ in Thousands	Common Stock	Additional Paid-In Capital	Accumulated Deficit	Total
Increase (Decrease) in Shareholders' (Deficit) Equity
Transition adjustment for ASC 606			$ 14,090	$ 14,090
Balance at Dec. 31, 2017	$ 1,325	$ 1,009,362	(1,028,582)	(17,895)
Balance (in shares) at Dec. 31, 2017	132,526
Increase (Decrease) in Shareholders' (Deficit) Equity
Net loss			(38,613)	(38,613)
Issuance of common stock pursuant to the exercise of stock options	$ 4	2,255		2,259
Issuance of common stock pursuant to the exercise of stock options (in shares)	421
Stock option and restricted stock compensation expense		3,746		3,746
Directors' deferred share units converted	$ 1	(1)
Directors' deferred share units converted (in shares)	77
Directors' deferred share unit compensation		102		102
Balance at Mar. 31, 2018	$ 1,330	1,015,464	(1,053,105)	(36,311)
Balance (in shares) at Mar. 31, 2018	133,024
Increase (Decrease) in Shareholders' (Deficit) Equity
Net loss			(41,624)	(41,624)
Issuance of common stock pursuant to the exercise of stock options	$ 1	558		559
Issuance of common stock pursuant to the exercise of stock options (in shares)	146
Issuance of common stock	$ 158	162,382		162,540
Issuance of common stock (in shares)	15,755
Stock option and restricted stock compensation expense		3,971		3,971
Directors' deferred share units converted	$ 1			1
Directors' deferred share units converted (in shares)	96
Directors' deferred share unit compensation		54		54
Balance at Jun. 30, 2018	$ 1,490	1,182,429	(1,094,729)	89,190
Balance (in shares) at Jun. 30, 2018	149,021
Increase (Decrease) in Shareholders' (Deficit) Equity
Net loss			(46,807)	(46,807)
Issuance of common stock pursuant to the exercise of stock options		124		124
Issuance of common stock pursuant to the exercise of stock options (in shares)	28
Issuance of common stock		(28)		(28)
Stock option and restricted stock compensation expense		4,308		4,308
Directors' deferred share unit compensation		102		102
Balance at Sep. 30, 2018	$ 1,490	1,186,935	(1,141,536)	46,889
Balance (in shares) at Sep. 30, 2018	149,049
Increase (Decrease) in Shareholders' (Deficit) Equity
Net loss			(41,799)	(41,799)
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan	$ 4	1,355		1,359
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan (in shares)	351
Stock option and restricted stock compensation expense		4,420		4,420
Directors' deferred share unit compensation		103		103
Balance at Dec. 31, 2018	$ 1,494	1,192,813	(1,183,335)	$ 10,972
Balance (in shares) at Dec. 31, 2018	149,400			149,400
Increase (Decrease) in Shareholders' (Deficit) Equity
Net loss			(43,751)	$ (43,751)
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan		68		68
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan (in shares)	25
Stock option and restricted stock compensation expense		5,007		5,007
Directors' deferred share unit compensation		100		100
Balance at Mar. 31, 2019	$ 1,494	$ 1,197,988	$ (1,227,086)	$ (27,604)
Balance (in shares) at Mar. 31, 2019	149,425			149,425