Development Derivative Liability - Narrative (Details)	1 Months Ended		3 Months Ended
	Mar. 31, 2020 USD ($) tranche $ / shares shares	Jan. 31, 2020 USD ($)	Mar. 31, 2020 USD ($) tranche $ / shares shares	Mar. 31, 2019 USD ($)	Dec. 31, 2019 USD ($) shares
Derivative [Line Items]
Loss from remeasurement of development derivative liability			$ (454,000)	$ 0
Common stock, shares issued (in shares) | shares	28,810,607		28,810,607		28,796,371
Common stock, shares issued, value	$ 29,000		$ 29,000		$ 29,000
SFJ Pharmaceuticals
Derivative [Line Items]
Cost of borrowing rate			2.50%
SFJ Agreement
Derivative [Line Items]
Potential milestones payment			$ 120,000,000
Cost of borrowing rate			16.00%
Collaborative Arrangement, Transaction with Party to Collaborative Arrangement | SFJ Agreement
Derivative [Line Items]
Common stock, shares issued (in shares) | shares	2,200,000		2,200,000
Shares issued (in dollars per share) | $ / shares	$ 6.50		$ 6.50
Common stock, shares issued term (in years)			10 years
Number of tranches | tranche	2		2
Common stock, shares issued, value	$ 7,900,000		$ 7,900,000
Collaborative Arrangement, Transaction with Party to Collaborative Arrangement | SFJ Agreement | Tranche A
Derivative [Line Items]
Common stock, shares issued (in shares) | shares	1,100,000		1,100,000
Collaborative Arrangement, Transaction with Party to Collaborative Arrangement | SFJ Agreement | Tranche B
Derivative [Line Items]
Common stock, shares issued (in shares) | shares	1,100,000		1,100,000
Collaborative Arrangement, Transaction with Party to Collaborative Arrangement | PB2452 | SFJ Agreement
Derivative [Line Items]
Initial payment received for license agreement	$ 10,000,000		$ 10,000,000
Additional payment received for license agreement	79,400,000.0
Additional payment for license	600,000
Collaborative Arrangement, Transaction with Party to Collaborative Arrangement | PB2452 | Maximum | SFJ Agreement
Derivative [Line Items]
Potential milestones payment receivable	$ 30,000,000.0	$ 120,000,000.0
Collaborative Arrangement, Transaction with Party to Collaborative Arrangement | PB2452 | United States Food And Drug Administration | SFJ Agreement
Derivative [Line Items]
Initial payment for license			5,000,000
Additional payment for license			325,000,000
Collaborative Arrangement, Transaction with Party to Collaborative Arrangement | PB2452 | European Medicines Agency | SFJ Agreement
Derivative [Line Items]
Initial payment for license			5,000,000
Additional payment for license			205,000,000
Collaborative Arrangement, Transaction with Party to Collaborative Arrangement | PB2452 | Pharmaceuticals And Medical Devices Agency Of Japan And National Medical Productions Administration Of China | SFJ Agreement
Derivative [Line Items]
Initial payment for license			1,000,000
Additional payment for license			59,000,000
Derivative
Derivative [Line Items]
Initial payment, net of common stock warrants			2,100,000
Funding during the period			600,000
Change in fair value			500,000
Derivative | Collaborative Arrangement, Transaction with Party to Collaborative Arrangement | SFJ Agreement
Derivative [Line Items]
Initial payment, net of common stock warrants			2,075,000
Funding during the period			557,000
Change in fair value			$ 454,000