Nature of the Business - Additional Information (Detail) - USD ($)			1 Months Ended	3 Months Ended				6 Months Ended
	May 07, 2019	Apr. 03, 2017	May 31, 2019	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018	Apr. 15, 2019	Dec. 31, 2018	Apr. 30, 2017
Summary Of Significant Accounting Policies [Line Items]
Proceeds from sale of equity securities								$ 48,500,000	$ 11,050,000
Proceeds from sale of research and development operations								2,025,000
Accumulated deficit				$ 543,165,000				543,165,000			$ 523,277,000
Net loss				9,430,000	$ 10,458,000	$ 17,769,000	$ 17,782,000	19,888,000	35,551,000
Cash used in operating activities								25,525,000	$ 33,582,000
Cash, cash equivalents and marketable securities				$ 39,900,000				$ 39,900,000
Silver Creek Pharmaceuticals, Inc. [Member]
Summary Of Significant Accounting Policies [Line Items]
Proceeds from sale of equity securities	$ 7,800,000		$ 7,800,000
Silver Creek Pharmaceuticals, Inc. [Member] | Laboratory Equipment [Member]
Summary Of Significant Accounting Policies [Line Items]
Proceeds from sale of research and development operations			$ 1,400,000
Loan Agreement [Member] | Hercules Capital, Inc [Member] | Term Loan [Member]
Summary Of Significant Accounting Policies [Line Items]
Payoff amount										$ 16,000,000
Ipsen [Member] | Ongoing Multi Part Clinical [Member]
Summary Of Significant Accounting Policies [Line Items]
Contingent milestone payments receivable		$ 5,000,000
Ipsen [Member] | First Line Treatment of Metastatic Adenocarcinoma of Pancreas [Member]
Summary Of Significant Accounting Policies [Line Items]
Contingent milestone payments receivable		225,000,000
Ipsen [Member] | After Failure of First Line Chemotherapy [Member]
Summary Of Significant Accounting Policies [Line Items]
Contingent milestone payments receivable		150,000,000
Ipsen [Member] | Additional Indication [Member]
Summary Of Significant Accounting Policies [Line Items]
Contingent milestone payments receivable		75,000,000
Ipsen [Member] | Asset Sale Agreement [Member] | Maximum [Member]
Summary Of Significant Accounting Policies [Line Items]
Contingent milestone payments receivable												$ 455,000,000
Additional payments receivable on achievement of certain milestone events		450,000,000
Servier [Member] | Asset Sale Agreement [Member] | Development and Commercialization Milestones [Member]
Summary Of Significant Accounting Policies [Line Items]
Maximum amount of milestone payments that can be received		33,000,000
Milestone payment received		$ 28,000,000