Composition of Total Revenue - Schedule of Net Product Sales by Geography (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Sales Information [Line Items]
Product sales, net	$ 116,178	$ 128,596	$ 136,851
ZEVALIN [Member]
Sales Information [Line Items]
Product sales, net	11,759	10,730	17,457
United States [Member]
Sales Information [Line Items]
Product sales, net	107,135	125,074	130,432
Total International [Member]
Sales Information [Line Items]
Product sales, net	9,043	3,522	6,419
Total International [Member] | Europe [Member] | ZEVALIN [Member]
Sales Information [Line Items]
Product sales, net	7,727	3,522	2,234
Total International [Member] | Asia Pacific [Member] | ZEVALIN [Member]
Sales Information [Line Items]
Product sales, net	$ 1,316	$ 0	$ 4,185
Geographic Concentration Risk [Member] | Sales [Member]
Sales Information [Line Items]
Percentage of sales	100.00%	100.00%	100.00%
Geographic Concentration Risk [Member] | Sales [Member] | United States [Member]
Sales Information [Line Items]
Percentage of sales	92.20%	97.30%	95.30%
Geographic Concentration Risk [Member] | Sales [Member] | Total International [Member]
Sales Information [Line Items]
Percentage of sales	7.80%	2.70%	4.70%
Geographic Concentration Risk [Member] | Sales [Member] | Total International [Member] | Europe [Member] | ZEVALIN [Member]
Sales Information [Line Items]
Percentage of sales	6.70%	2.70%	1.60%
Geographic Concentration Risk [Member] | Sales [Member] | Total International [Member] | Asia Pacific [Member] | ZEVALIN [Member]
Sales Information [Line Items]
Percentage of sales	1.10%	0.00%	3.10%