Stockholders' Equity - Changes in Stockholders' Equity (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018
Increase (Decrease) in Stockholders' Equity
Period Start Balance	$ 109,626	$ 112,693	$ 124,060	$ 128,047	$ 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	45		374
Shares Issued in Connection with Cantor Agreement	2,463	7,212	4,151	5,493	8,271	11,689
Shares Issued in Connection with Kolltan Severance				8	19	38
Share-Based Compensation	707	1,464	1,693	1,793	2,048	2,488
Unrealized Gain/(Loss) on Marketable Securities	(3)	36	19	(8)	31	(5)	$ 52	18
Net Loss	(11,413)	(11,779)	(17,239)	(7,243)	(16,407)	(118,131)	(40,431)	(141,781)
Period End Balance	$ 101,380	109,626	112,693	128,135	128,047	134,085	$ 101,380	128,135
Balance (in shares)	15,904,520						15,904,520
ASC 606
Increase (Decrease) in Stockholders' Equity
Adoption of ASC 606						1,263
Common Stock
Increase (Decrease) in Stockholders' Equity
Period Start Balance	$ 15	$ 13	$ 12	$ 10	$ 9	$ 9	$ 12	$ 9
Balance (in shares)	14,816,917	12,844,711	11,957,635	10,443,058	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	7,038		9,453
Shares Cancelled under Stock Option and Employee Stock Purchase Plans (in shares)		(222)
Shares Issued in Connection with Cantor Agreement	$ 1	$ 2	$ 1	$ 1	$ 1
Shares Issued in Connection with Cantor Agreement (in shares)	1,087,603	1,972,428	883,569	787,163	884,068	312,802	3,900,000
Shares Issued in Connection with Kolltan Severance (in shares)				1,108	1,071	971
Period End Balance	$ 16	$ 15	$ 13	$ 11	$ 10	$ 9	$ 16	$ 11
Balance (in shares)	15,904,520	14,816,917	12,844,711	11,238,367	10,443,058	9,557,919	15,904,520	11,238,367
Additional Paid-In Capital
Increase (Decrease) in Stockholders' Equity
Period Start Balance	$ 1,098,429	$ 1,089,755	$ 1,083,903	$ 1,071,239	$ 1,060,902	$ 1,046,313	$ 1,083,903	$ 1,046,313
Shares Issued under Stock Option and Employee Stock Purchase Plans			9	45		374
Shares Issued in Connection with Cantor Agreement	2,462	7,210	4,150	5,492	8,270	11,689
Shares Issued in Connection with Kolltan Severance				8	19	38
Share-Based Compensation	707	1,464	1,693	1,793	2,048	2,488
Period End Balance	1,101,598	1,098,429	1,089,755	1,078,577	1,071,239	1,060,902	1,101,598	1,078,577
Accumulated Other Comprehensive Income.
Increase (Decrease) in Stockholders' Equity
Period Start Balance	2,638	2,602	2,583	2,590	2,559	2,564	2,583	2,564
Unrealized Gain/(Loss) on Marketable Securities	(3)	36	19	(8)	31	(5)
Period End Balance	2,635	2,638	2,602	2,582	2,590	2,559	2,635	2,582
Accumulated Deficit
Increase (Decrease) in Stockholders' Equity
Period Start Balance	(991,456)	(979,677)	(962,438)	(945,792)	(929,385)	(812,517)	(962,438)	(812,517)
Net Loss	(11,413)	(11,779)	(17,239)	(7,243)	(16,407)	(118,131)
Period End Balance	$ (1,002,869)	$ (991,456)	$ (979,677)	$ (953,035)	$ (945,792)	(929,385)	$ (1,002,869)	$ (953,035)
Accumulated Deficit | ASC 606
Increase (Decrease) in Stockholders' Equity
Adoption of ASC 606						$ 1,263