Balance Sheet Account Detail - Deferred Revenue (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
Disaggregation of Revenue [Line Items]
Deferred revenue	$ 4,850
Deferred revenue			$ 4,187
Increase (decrease) in retained earnings due to revenue recognition	(599,886)		(502,107)
EVOMELA Out-License in China Territory [Member]
Disaggregation of Revenue [Line Items]
Deferred revenue	4,850
EVOMELA [Member]
Disaggregation of Revenue [Line Items]
Deferred revenue			3,800
Deferred revenue			3,819
ZEVALIN, in India Territory [Member]
Disaggregation of Revenue [Line Items]
Deferred revenue			400
Deferred revenue			$ 368
Difference between Revenue Guidance in Effect before and after Topic 606 [Member] | Accounting Standards Update 2014-09 [Member]
Disaggregation of Revenue [Line Items]
Increase (decrease) in retained earnings due to revenue recognition	$ (3,087)	$ 4,700