The Company and Basis of Presentation - Additional Information (Detail) - USD ($)	1 Months Ended		3 Months Ended		6 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2016	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2016	Dec. 31, 2017
Class of Stock [Line Items]
Proceeds from issuance of common stock, net of at-the-market offering commissions					$ 0	$ 29,100,000
Company's incorporation date					Aug. 10, 1999
Commencement date of operations					Jun. 30, 2002
Change of entity name date					Apr. 19, 2005
Net loss			$ 34,080,000	$ 26,874,000	$ 69,118,000	54,029,000
Net cash used in operating activities					47,701,000	$ 45,482,000
Working capital surplus			214,600,000		214,600,000
Accumulated deficit			611,284,000		611,284,000			$ 542,167,000
Cash, cash equivalents, and short-term investments			$ 233,700,000		$ 233,700,000			$ 282,900,000
Cash and cash equivalents, and investments, expected funding term for operating plan					12 months
At the Market Offering
Class of Stock [Line Items]
Aggregate authorized offering price		$ 75,000,000.0
Commission (percent)		3.00%
Issuance of common stock (in shares)						1,413,051
Proceeds from issuance of common stock, net of at-the-market offering commissions						$ 28,600,000
Proceeds from issuance of common stock, net of commissions and offering costs						29,100,000
Stock offering expenses						500,000
Stock offering expenses paid							$ 200,000
Unpaid stock offering costs				$ 100,000		$ 100,000
Controlled Equity Offering
Class of Stock [Line Items]
Aggregate authorized offering price	$ 125,000,000.0
Weighted Average | At the Market Offering
Class of Stock [Line Items]
Offering price per share (in dollars per share)				$ 21.23		$ 21.23