Organization and Significant Accounting Policies - Additional Information (Details) $ in Thousands			1 Months Ended	3 Months Ended
	Mar. 26, 2020 USD ($) shares	Mar. 03, 2020 USD ($) shares	Mar. 31, 2020 USD ($)	Mar. 31, 2020 USD ($) segment shares	Mar. 31, 2019 USD ($)	Dec. 31, 2019 USD ($)
Organization And Significant Accounting Policies [Line Items]
Accumulated deficit | $			$ 226,151	$ 226,151		$ 219,984
Proceeds from issuance of common stock and pre-funded warrants, net of underwriting fees and commissions | $				12,577	$ 0
Proceeds from exercise of common stock warrants | $				2,880	0
Cash and cash equivalents | $			21,785	$ 21,785	$ 6,077	$ 13,711
Common stock issued during period (in shares) | shares	10,550,000	14,000,000
Period of option to purchase additional shares		30 days
Common stock, maximum additional shares available for purchase (in shares) | shares		2,750,000
Maximum additional number of common warrants available for purchase (in shares) | shares		2,750,000
Percentage of common stock sold related to warrants issued	3.00%	3.00%
Number of segment | segment				1
Percentage of total revenue generated from Japan licensing partner				84.00%	100.00%
Common Stock
Organization And Significant Accounting Policies [Line Items]
Common stock issued during period (in shares) | shares		1,498,602
License and collaboration revenue
Organization And Significant Accounting Policies [Line Items]
Revenue | $				$ 1,024	$ 1,100
License and collaboration revenue | Amended Sato Agreement
Organization And Significant Accounting Policies [Line Items]
Revenue | $				1,024	$ 1,100
Underwriter Warrant
Organization And Significant Accounting Policies [Line Items]
Warrants issued (in shares) | shares		594,958
Pre Funded Warrant
Organization And Significant Accounting Policies [Line Items]
Warrants issued (in shares) | shares	8,054,652	4,333,334
Common Warrant
Organization And Significant Accounting Policies [Line Items]
Warrants issued (in shares) | shares		2,750,000
Common Warrant | Maximum
Organization And Significant Accounting Policies [Line Items]
Warrants issued (in shares) | shares		18,333,334
Placement Agent Warrant | Maximum
Organization And Significant Accounting Policies [Line Items]
Warrants issued (in shares) | shares	558,140
March 2020 Public Offering
Organization And Significant Accounting Policies [Line Items]
Proceeds from issuance of common stock and pre-funded warrants, net of underwriting fees and commissions | $		$ 5,158
Underwriting discounts and commissions and offering expenses | $		$ 791
March 2020 Public Offering | Pre Funded Warrant
Organization And Significant Accounting Policies [Line Items]
Proceeds from exercise of common stock warrants | $				$ 2,880
Public Offering | Pre Funded Warrant | Common Stock
Organization And Significant Accounting Policies [Line Items]
Common stock issued during period (in shares) | shares				15,498,602
March 2020 Registered Direct Offering
Organization And Significant Accounting Policies [Line Items]
Proceeds from issuance of common stock and pre-funded warrants, net of underwriting fees and commissions | $	$ 7,225
Underwriting discounts and commissions and offering expenses | $	$ 774
March 2020 Registered Direct Offering | Pre Funded Warrant | Common Stock
Organization And Significant Accounting Policies [Line Items]
Common stock issued during period (in shares) | shares				10,550,000
Common Stock Purchase Agreement | Common Stock
Organization And Significant Accounting Policies [Line Items]
Common stock issued during period (in shares) | shares				700,000
March 2020 Offerings
Organization And Significant Accounting Policies [Line Items]
Net proceeds from March 2020 offerings and related warrant exercises | $			$ 15,263