Investments, Carried at Cost - Details of Investments Carried at Cost (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Nov. 18, 2016
Investment [Line Items]
Investments, carried at cost	$ 35,187	$ 35,187
AdvisorEngine [Member] | Convertible Preferred Stock [Member]
Investment [Line Items]
Investments, carried at cost	25,000	25,000	$ 20,000
AdvisorEngine [Member] | Equity Option [Member]
Investment [Line Items]
Investments, carried at cost	3,278	3,278
Thesys Group, Inc [Member]
Investment [Line Items]
Investments, carried at cost	$ 6,909	$ 6,909