Organization and Significant Accounting Policies - Additional Information (Details)			4 Months Ended	5 Months Ended	12 Months Ended
	Jan. 31, 2020 shares	Aug. 30, 2019 USD ($) $ / shares shares	Dec. 31, 2019 USD ($) $ / shares shares	Jan. 31, 2020 $ / shares shares	Dec. 31, 2019 USD ($) segment	Dec. 31, 2018 USD ($)	Feb. 01, 2020 $ / shares	Sep. 30, 2019 USD ($)	Aug. 29, 2019 $ / shares	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Jan. 01, 2019 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)
Organization And Significant Accounting Policies [Line Items]
Accumulated deficit			$ 219,984,000		$ 219,984,000	$ 188,893,000		$ 214,036,000		$ 204,540,000	$ 196,244,000		$ 185,213,000	$ 176,718,000	$ 168,429,000
Cash and cash equivalents			13,711,000		13,711,000	8,194,000		15,549,000		22,438,000	6,077,000		12,173,000	20,984,000	28,100,000
Impairment of long-lived assets					$ 0	0
Life of shelf registration period					3 years
Accrued interest or penalties related to uncertain tax positions			0		$ 0	0
Number of segment | segment					1
Property and equipment, gross			16,185,000		$ 16,185,000	20,456,000
Facility lease obligation			0		0	7,998,000							$ 7,998,000	$ 7,998,000	$ 7,998,000
Right-of-use lease assets capitalized			1,833,000		1,833,000	0		$ 1,837,000		$ 1,837,000	$ 1,833,000
Lease liabilities capitalized			$ 6,262,000		6,262,000
Amended Sato Agreement
Organization And Significant Accounting Policies [Line Items]
Revenue					$ 4,477,000	$ 5,982,000
Percentage of total revenue generated from Japan licensing partner					91.00%	100.00%
Subsequent event
Organization And Significant Accounting Policies [Line Items]
Closing sale price of common stock (in usd per share) | $ / shares							$ 0.52
Aspire Capital | Common Stock Purchase Agreement
Organization And Significant Accounting Policies [Line Items]
Maximum value of shares of common stock authorized to be sold		$ 25,000,000
Common stock purchase agreement term		30 months
Maximum number of shares that may be sold under agreement (in shares) | shares		5,211,339
Percentage of common stock on date of purchase agreement that cannot be exceeded		19.99%
Closing sale price of common stock (in usd per share) | $ / shares									$ 2.17
Minimum closing sale price to effect purchase of shares (in usd per share) | $ / shares		$ 0.25
Common stock issued during period (in shares) | shares			300,000
Average sales price per share of common stock (in usd per share) | $ / shares			$ 2.49
Shares of common stock issued for commitment fee (in shares) | shares		345,622
Aspire Capital | Common Stock Purchase Agreement | Subsequent event
Organization And Significant Accounting Policies [Line Items]
Common stock issued during period (in shares) | shares	1,345,622			1,000,000
Average sales price per share of common stock (in usd per share) | $ / shares				$ 1.19
Cumulative Effect, Period of Adoption, Adjustment
Organization And Significant Accounting Policies [Line Items]
Increase to accumulated deficit												$ 714,000
ASU 2016-02 (Topic 842)
Organization And Significant Accounting Policies [Line Items]
Facility lease obligation												7,998,000
Right-of-use lease assets capitalized												1,827,000
Lease liabilities capitalized												6,786,000
ASU 2016-02 (Topic 842) | Cumulative Effect, Period of Adoption, Adjustment
Organization And Significant Accounting Policies [Line Items]
Increase to accumulated deficit												$ 714,000