Out-License Of Zevalin In Certain Ex-U.S. Territories (Detail) - USD ($) $ in Thousands		1 Months Ended			2 Months Ended	3 Months Ended			6 Months Ended
	Jun. 30, 2017	Apr. 30, 2018	Jan. 31, 2016	Dec. 31, 2015	Jan. 31, 2016	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2015	Jun. 30, 2018	Jun. 30, 2017
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone net sales achievement						$ 24,168	$ 34,301		$ 54,662	$ 63,402
Zevalin Rights [Member] | Total Consideration [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total consideration received					$ 18,000
Zevalin Rights [Member] | Payment One [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total consideration received				$ 15,000
Zevalin Rights [Member] | Payment Two [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total consideration received			$ 3,000
License and Service [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone net sales achievement						$ 415	$ 3,145		$ 2,799	$ 6,401
License and Service [Member] | Zevalin Rights [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone net sales achievement		$ 2,000
License and Service [Member] | Zevalin Rights [Member] | Total Consideration [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone net sales achievement								$ 15,000
License and Service [Member] | Zevalin Rights [Member] | Payment Two [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone net sales achievement	$ 3,000