Stockholders' Equity - Changes in Stockholders' equity (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Increase (Decrease) in Stockholders' Equity
Period Start Balance	$ 112,693	$ 124,060	$ 134,085	$ 236,369	$ 124,060	$ 236,369
Balance (in shares)		11,957,635			11,957,635
Shares Issued under Stock Option and Employee Stock Purchase Plans		$ 9		374
Shares Issued in Connection with Cantor Agreement	7,212	4,151	8,271	11,689
Shares Issued in Connection with Kolltan Severance			19	38
Share-Based Compensation	1,464	1,693	2,048	2,488
Unrealized Gain/(Loss) on Marketable Securities	36	19	31	(5)	$ 55	26
Net Loss	(11,779)	(17,239)	(16,407)	(118,131)	(29,018)	(134,538)
Period End Balance	$ 109,626	112,693	128,047	134,085	$ 109,626	128,047
Balance (in shares)	14,816,917				14,816,917
ASC 606
Increase (Decrease) in Stockholders' Equity
Adoption of ASC 606				1,263
Common Stock
Increase (Decrease) in Stockholders' Equity
Period Start Balance	$ 13	$ 12	$ 9	$ 9	$ 12	$ 9
Balance (in shares)	12,844,711	11,957,635	9,557,919	9,234,693	11,957,635	9,234,693
Shares Issued under Stock Option and Employee Stock Purchase Plans (in shares)		3,507		9,453
Shares Cancelled under Stock Option and Employee Stock Purchase Plans (in shares)	(222)
Shares Issued in Connection with Cantor Agreement	$ 2	$ 1	$ 1
Shares Issued in Connection with Cantor Agreement (in shares)	1,972,428	883,569	884,068	312,802	2,856,194
Shares Issued in Connection with Kolltan Severance (in shares)			1,071	971
Period End Balance	$ 15	$ 13	$ 10	$ 9	$ 15	$ 10
Balance (in shares)	14,816,917	12,844,711	10,443,058	9,557,919	14,816,917	10,443,058
Additional Paid-In Capital
Increase (Decrease) in Stockholders' Equity
Period Start Balance	$ 1,089,755	$ 1,083,903	$ 1,060,902	$ 1,046,313	$ 1,083,903	$ 1,046,313
Shares Issued under Stock Option and Employee Stock Purchase Plans		9		374
Shares Issued in Connection with Cantor Agreement	7,210	4,150	8,270	11,689
Shares Issued in Connection with Kolltan Severance			19	38
Share-Based Compensation	1,464	1,693	2,048	2,488
Period End Balance	1,098,429	1,089,755	1,071,239	1,060,902	1,098,429	1,071,239
Accumulated Other Comprehensive Income.
Increase (Decrease) in Stockholders' Equity
Period Start Balance	2,602	2,583	2,559	2,564	2,583	2,564
Unrealized Gain/(Loss) on Marketable Securities	36	19	31	(5)
Period End Balance	2,638	2,602	2,590	2,559	2,638	2,590
Accumulated Deficit
Increase (Decrease) in Stockholders' Equity
Period Start Balance	(979,677)	(962,438)	(929,385)	(812,517)	(962,438)	(812,517)
Net Loss	(11,779)	(17,239)	(16,407)	(118,131)
Period End Balance	$ (991,456)	$ (979,677)	$ (945,792)	(929,385)	$ (991,456)	$ (945,792)
Accumulated Deficit | ASC 606
Increase (Decrease) in Stockholders' Equity
Adoption of ASC 606				$ 1,263