Cash, Cash Equivalents and Investments - Additional Information (Detail) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents maturities	90 days	90 days
Investment of cash equivalents are redeemable upon demand using amortized cost method	$ 1.00
Investment in money market funds restricted by investment in short term securities, percentage	100.00%
Cash	$ 3,146,000	$ 3,785,000
Cash equivalents	9,306,000	1,774,000
Cash and cash equivalents in foreign bank	1,300,000	1,200,000
Short-term investments	$ 22,285,000	32,499,000
Short-term investments, maturities	90 days
Net unrealized gains (losses)	$ 36,000	$ 34,000
Percentage of investment in municipal bond protected by bond default insurance		8.00%
Corporate Bonds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash equivalents	5,600,000	$ 1,300,000
Short-term investments	9,500,000	18,500,000
Certificates of Deposit [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash equivalents	1,900,000
Short-term investments	8,500,000	7,400,000
U.S. Government Agency Bonds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash equivalents	1,700,000	300,000
Short-term investments	4,300,000	4,500,000
Money Market Funds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash equivalents	100,000	200,000
Pre-refunded Municipal Bonds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term investments		2,100,000
China [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash in foreign bank	1,200,000	$ 1,000,000
Maximum [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Federal Deposit Insurance Corporation insured limit	$ 250,000