Organization and Significant Accounting Policies (Detail)	3 Months Ended		12 Months Ended
	Mar. 31, 2018 USD ($) segment	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)	Feb. 26, 2018 USD ($)
Debt Instrument [Line Items]
Number of operating segments | segment	1
Cash, cash equivalents and short-term investments	$ 72,800,000
Accounts receivable outstanding	182,000		$ 17,144,000
Current liabilities	24,630,000		26,307,000
Net loss	30,054,000	$ 15,089,000	53,500,000
Net cash used in operating activities	$ 11,296,000	$ 12,702,000	$ 36,200,000
CRG Loan
Debt Instrument [Line Items]
Additional increase in credit				$ 45,000,000.0