Description of Business and Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)	1 Months Ended				3 Months Ended		9 Months Ended		33 Months Ended
	Sep. 30, 2017	Jul. 31, 2017	Apr. 30, 2017	Aug. 31, 2013	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2017	Mar. 31, 2017	Mar. 31, 2016
Description Of Business And Summary Of Significant Accounting Policies [Line Items]
Cash and cash equivalents					$ 47,338,000	$ 69,996,000	$ 47,338,000	$ 69,996,000	$ 47,338,000	$ 62,751,000	$ 62,091,000
Accumulated deficit					$ (226,671,000)		(226,671,000)		$ (226,671,000)	$ (199,317,000)
Cash flow from operations							$ 23,246,000	21,959,000
Issuance of common stock					109,322,626		109,322,626		109,322,626	104,551,466
Warrants expiration date							Mar. 31, 2017
Deferred revenue					$ 857,000		$ 857,000		$ 857,000	$ 582,000
Revenue recognized under grants					260,000	9,000	409,000	21,000
Cost of revenues					192,000	212,000	747,000	773,000
Dilutive effect					0	0	$ 0	$ 0
Common stock equivalents excluded from computing diluted net loss per share							14,000,000	12,700,000
NIH Research Grants Two [Member]
Description Of Business And Summary Of Significant Accounting Policies [Line Items]
Revenue recognized under grants		$ 1,657,000
Revenue recognized under grants					260,000		$ 409,000
Private, Not-for-Profit Organization [Member]
Description Of Business And Summary Of Significant Accounting Policies [Line Items]
Revenue recognized under grants					0	9,000	0	$ 21,000
Maximum [Member] | Private, Not-for-Profit Organization [Member]
Description Of Business And Summary Of Significant Accounting Policies [Line Items]
Revenue recognized under grants				$ 251,700
Research and Development Services [Member]
Description Of Business And Summary Of Significant Accounting Policies [Line Items]
Deferred revenue					900,000		900,000		900,000	$ 640,000
Research and Development Services Six [Member]
Description Of Business And Summary Of Significant Accounting Policies [Line Items]
Revenue related to research contract					0	7,000	0	32,000
Research and Development Services Four [Member]
Description Of Business And Summary Of Significant Accounting Policies [Line Items]
Revenue related to research contract					0	117,000	150,000	117,000	$ 620,000
Research and Development Services Five [Member]
Description Of Business And Summary Of Significant Accounting Policies [Line Items]
Revenue related to research contract					0	302,000	0	835,000
Research and Development Services Seven [Member]
Description Of Business And Summary Of Significant Accounting Policies [Line Items]
Revenue related to research contract					18,000	17,000	53,000	17,000
Research and Development Services Eight [Member]
Description Of Business And Summary Of Significant Accounting Policies [Line Items]
Revenue related to research contract					10,000	$ 0	29,000	$ 0
Research and Development Services Ten [Member]
Description Of Business And Summary Of Significant Accounting Policies [Line Items]
Revenue related to research contract					14,000		28,000
Research and Development Services Eleven [Member]
Description Of Business And Summary Of Significant Accounting Policies [Line Items]
Revenue related to research contract					18,750		56,250
Non-exclusive patent license agreement annual fee			$ 75,000
Non-exclusive patent license agreement term			2 years
Proceeds from Non-exclusive patent annual license fee			$ 75,000
Research and Development Services Nine [Member]
Description Of Business And Summary Of Significant Accounting Policies [Line Items]
Revenue related to research contract	$ 50,000				$ 0		$ 50,000
At-The-Market Facility [Member]
Description Of Business And Summary Of Significant Accounting Policies [Line Items]
Issuance of common stock					3,793,758		3,793,758		3,793,758
Gross proceeds from sale of common stock shares							$ 7,100,000