Financial Commitments & Contingencies and Key License Agreements - Additional Information (Details) - USD ($) $ in Thousands	1 Months Ended			12 Months Ended
	Apr. 30, 2019	Apr. 30, 2018	Apr. 30, 2016	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2019
Long-term Purchase Commitment [Line Items]
Lease cost				$ 2,207
Operating lease, rent expense					$ 1,600	$ 1,600
Operating lease, right-of-use assets				3,806
Operating lease, liability				4,055
Right-of-use asset obtained in exchange for operating lease liability				1,100
Payments to acquire intangible assets				0	2,650	$ 0
Deferred compensation liability, current and noncurrent				8,700	$ 6,200
SPI-2012
Long-term Purchase Commitment [Line Items]
Issuance (shares)			318,750
Spi Two Thousand Twelve
Long-term Purchase Commitment [Line Items]
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
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
ImmunGene, Inc.
Long-term Purchase Commitment [Line Items]
Potential payments based on achievement of regulatory milestones				26,100
Payments to acquire intangible assets	$ 2,800
Potential payments based on additional achievements of regulatory milestones				$ 5,000
Minimum
Long-term Purchase Commitment [Line Items]
Lessee, operating lease, remaining lease term				1 year
Maximum
Long-term Purchase Commitment [Line Items]
Lessee, operating lease, remaining lease term				4 years
Accounting Standards Update 2016-02
Long-term Purchase Commitment [Line Items]
Operating lease, right-of-use assets							$ 4,200
Operating lease, liability				$ 1,100			$ 4,200