Interim Consolidated Financial Statements - Additional Information (Detail) - USD ($)		3 Months Ended			6 Months Ended		12 Months Ended
	Jan. 02, 2018	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Summary Of Significant Accounting Policy [Line Items]
Deferred revenue		$ 143,023,000			$ 143,023,000		$ 145,207,000
Prepaid expenses and other current assets		12,789,000			12,789,000		18,942,000
Deferred commissions		12,514,000			12,514,000		13,731,000
Other assets		7,756,000			7,756,000		6,231,000
Retained earnings		125,698,000			125,698,000		123,010,000
Total assets		326,142,000			326,142,000		345,200,000
Total revenues		96,353,000		$ 89,733,000	174,102,000	$ 166,927,000
Net income		$ 7,788,000		$ 6,064,000	$ 6,055,000	$ 9,094,000
Diluted net income per share		$ 0.43		$ 0.34	$ 0.33	$ 0.50
ASU No. 2018-02 [Member]
Summary Of Significant Accounting Policy [Line Items]
Reclassification adjustment to retained earnings for tax effects	$ 26,000
ASC 606 [Member] | Initial Application Period Cumulative Effect Transition [Member]
Summary Of Significant Accounting Policy [Line Items]
Deferred revenue		$ 7,800,000			$ 7,800,000
Prepaid expenses and other current assets		(5,500,000)			(5,500,000)
Deferred commissions		900,000			900,000
Other assets		600,000			600,000
Retained earnings		3,800,000			3,800,000
ASC 606 [Member] | Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
Summary Of Significant Accounting Policy [Line Items]
Deferred revenue		(9,000,000)			(9,000,000)
Total assets		(7,000,000)			(7,000,000)
Total revenues		(100,000)			(2,400,000)
Net income		$ (300,000)			$ (1,800,000)
Diluted net income per share		$ 0.01			$ 0.10
Research Services [Member]
Summary Of Significant Accounting Policy [Line Items]
Out-of-period correction revenue		$ 1,000,000
Out-of-period correction revenue, after tax		700,000
Total revenues		$ 58,300,000		$ 54,575,000	$ 110,000,000	$ 106,318,000
Understatement of Revenue from Reprint Product Line [Member]
Summary Of Significant Accounting Policy [Line Items]
Out-of-period correction revenue			$ 800,000				200,000
Out-of-period correction revenue, after tax			$ 500,000				$ 100,000