Financial Commitments & Contingencies and Key License Agreements - Additional Information (Details) $ in Thousands	1 Months Ended			3 Months Ended		9 Months Ended
	Apr. 30, 2019 USD ($)	Apr. 30, 2018 USD ($)	Apr. 30, 2016 shares	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) product	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)	Jan. 01, 2019 USD ($)
Long-term Purchase Commitment [Line Items]
Lease cost				$ 569	$ 479	$ 1,758	$ 1,584
Operating lease right-of-use assets - non-current				2,662		2,662		$ 3,806
Operating lease liability				2,865		$ 2,865		4,055
Right-of-use asset obtained in exchange for operating lease liability							$ 4,600
Licensors | product						2
Deferrals and contributions				8,400		$ 8,400		8,700
Office equipment
Long-term Purchase Commitment [Line Items]
Operating lease right-of-use assets - non-current				300		300		400
Office and research facilities
Long-term Purchase Commitment [Line Items]
Operating lease right-of-use assets - non-current				$ 2,300		2,300		$ 3,400
ImmunGene
Long-term Purchase Commitment [Line Items]
Potential payments based on achievement of regulatory milestones						26,100
Asset purchase agreement, upfront payment	$ 2,800
Potential payments based on additional achievements of regulatory milestones						5,000
MD Anderson
Long-term Purchase Commitment [Line Items]
Potential payments based on achievement of regulatory milestones						6,000
Potential payments based on achievement of sales milestones						24,000
Payment Of Upfront Fee		$ 500
SPI-2012
Long-term Purchase Commitment [Line Items]
Issuance (shares) | shares			318,750
Potential payments based on achievement of regulatory milestones						10,000
Potential payments based on achievement of sales milestones						120,000
Poziotinib | Licensing Agreements
Long-term Purchase Commitment [Line Items]
Potential payments based on achievement of regulatory milestones						33,000
Potential payments based on achievement of sales milestones						$ 325,000
ASU 2016-02
Long-term Purchase Commitment [Line Items]
Operating lease right-of-use assets - non-current									$ 4,200
Operating lease liability									$ 4,200
Minimum
Long-term Purchase Commitment [Line Items]
Remaining lease term				1 year		1 year
Maximum
Long-term Purchase Commitment [Line Items]
Remaining lease term				4 years		4 years