Description of Business and Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)	6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2018	Jan. 01, 2019	Dec. 31, 2017
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Highly liquid marketable securities with original maturities			90 days or less
Cash equivalents include commercial paper and money market funds			$ 25,543,000		$ 39,761,000
Short-term marketable securities maturity description			Over 90 days that mature in less than one-year
Marketable securities other-than temporary impairments			$ 0
Credit exposure to any one financial institution or type of financial instrument			5% of its cash equivalents and marketable securities or $1,000, whichever is greater
Percentage of credit exposure to any one financial institution or type of financial instrument			5.00%
Maximum amount of credit exposure to any one financial institution or type of financial instrument			$ 1,000,000
Credit exposure limits of cash and cash equivalents and marketable securities			40% of its cash equivalents and marketable securities, or $15,000, whichever is greater
Percentage of credit exposure limits based on cash and cash equivalents and marketable securities			40.00%
Credit exposure limits of cash and cash equivalents and marketable securities under option two			$ 15,000,000
Purchase price adjustment date			1 year
Impairment of goodwill	$ 0	$ 0
Term of patent			17 years
Increase to opening retained earnings			$ 139,000
Subsequent Event [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Estimated right of use assets				$ 2,700,000
Estimated lease liabilities				$ 3,800,000
ASU No. 2014-09 [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Increase to opening retained earnings			$ 139,000