Significant Accounting Policies - Contingent Consideration (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Business Combination Contingent Liability [Roll Forward]
Beginning balance		$ 45,100
Provisional purchase price accounting adjustment (Note 3)		(14,600)	$ 0	$ 0
Loss from change in fair value of contingent consideration		9,100	(1,100)	$ 0
Ending balance		$ 39,600	$ 45,100
Discounted Cash Flow Approach | Vicinium | Minimum
Business Combination Contingent Liability [Roll Forward]
Discount rate (percentage)		8.30%
Discounted Cash Flow Approach | Vicinium | Maximum
Business Combination Contingent Liability [Roll Forward]
Discount rate (percentage)		10.50%	11.50%
Unobservable Inputs (Level 3)
Business Combination Contingent Liability [Roll Forward]
Beginning balance		$ 45,100
Ending balance		39,600	$ 45,100
Unobservable Inputs (Level 3) | Discounted Cash Flow Approach | Vicinium | Minimum
Business Combination Contingent Liability [Roll Forward]
Discount rate (percentage)			9.30%
Viventia Bio Inc.
Business Combination Contingent Liability [Roll Forward]
Provisional purchase price accounting adjustment (Note 3)	$ (14,600)
Viventia Bio Inc. | Unobservable Inputs (Level 3)
Business Combination Contingent Liability [Roll Forward]
Beginning balance		45,100
Provisional purchase price accounting adjustment (Note 3)		(14,600)
Loss from change in fair value of contingent consideration		9,100
Ending balance		$ 39,600	$ 45,100