STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Income	Accumulated Deficit ASC 606	Accumulated Deficit	ASC 606	Total
Period Start Balance at Dec. 31, 2015	$ 7	$ 878,747	$ 2,307		$ (590,956)		$ 290,105
Balance (in shares) at Dec. 31, 2015	6,579,040
Increase (Decrease) in Stockholders' Equity
Shares Issued under Stock Option and Employee Stock Purchase Plans		535					535
Shares Issued under Stock Option and Employee Stock Purchase Plans (in shares)	10,543
Shares Issued in Connection with Cantor Agreement		13,946					13,946
Shares Issued in Connection with Cantor Agreement (in shares)	220,253
Shares Issued in Connection with the Kolltan Acquisition	$ 1	73,396					73,397
Shares Issued in Connection with the Kolltan Acquisition (in shares)	1,217,200
Shares Issued in Connection with Kolltan Severance		427					427
Shares Issued in Connection with Kolltan Severance (in shares)	7,407
Share-Based Compensation		15,317					15,317
Unrealized Gains on Marketable Securities			234				234
Net Loss					(128,530)		(128,530)
Period End Balance at Dec. 31, 2016	$ 8	982,368	2,541		(719,486)		265,431
Balance (in shares) at Dec. 31, 2016	8,034,443
Increase (Decrease) in Stockholders' Equity
Shares Issued under Stock Option and Employee Stock Purchase Plans		265					265
Shares Issued under Stock Option and Employee Stock Purchase Plans (in shares)	11,581
Shares Issued in Connection with Cantor Agreement	$ 1	51,024					51,025
Shares Issued in Connection with Cantor Agreement (in shares)	1,181,524
Shares Issued in Connection with Kolltan Severance		344					344
Shares Issued in Connection with Kolltan Severance (in shares)	7,145
Share-Based Compensation		12,312					12,312
Unrealized Gains on Marketable Securities			23				23
Net Loss					(93,031)		(93,031)
Period End Balance at Dec. 31, 2017	$ 9	1,046,313	2,564		(812,517)		$ 236,369
Balance (in shares) at Dec. 31, 2017	9,234,693						9,234,693
Increase (Decrease) in Stockholders' Equity
Shares Issued under Stock Option and Employee Stock Purchase Plans		419					$ 419
Shares Issued under Stock Option and Employee Stock Purchase Plans (in shares)	16,047
Shares Issued in Connection with Cantor Agreement	$ 3	29,019					29,022
Shares Issued in Connection with Cantor Agreement (in shares)	2,702,660
Shares Issued in Connection with Kolltan Severance		71					71
Shares Issued in Connection with Kolltan Severance (in shares)	4,235
Share-Based Compensation		8,081					8,081
Unrealized Gains on Marketable Securities			19				19
Adoption of ASC 606				$ 1,263		$ 1,263
Net Loss					(151,184)		(151,184)
Period End Balance at Dec. 31, 2018	$ 12	$ 1,083,903	$ 2,583		$ (962,438)		$ 124,060
Balance (in shares) at Dec. 31, 2018	11,957,635						11,957,635