Summary Selected Quarterly Financial Data - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017
Research Services [Member]
Quantifying Misstatement In Current Year Financial Statements [Line Items]
Out-of-period correction revenue	$ 1.0
Out-of-period correction revenue, after tax	$ 0.7
Understatement of Revenue from Reprint Product Line [Member]
Quantifying Misstatement In Current Year Financial Statements [Line Items]
Out-of-period correction revenue		$ 0.8	$ 0.2
Out-of-period correction revenue, after tax		$ 0.5	$ 0.1