Fair Value Measurements - Summary of Categorization of Assets and Liabilities Measured at Fair Value (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Liabilities:
Deferred consideration (Note 12)	$ 173,024	$ 161,540
Fair Value, Measurements, Recurring [Member]
Assets:
Cash equivalents	317	24
Securities owned, at fair value	17,319	8,873
Total	17,636	8,897
Liabilities:
Deferred consideration (Note 12)	173,024	161,540
Securities sold, but not yet purchased	582	1,698
Total	173,606	163,238
Fair Value, Measurements, Nonrecurring [Member]
Assets:
Thesys Group, Inc. – Series Y preferred stock		3,080
AdvisorEngine – Financial Interests	28,172
Total	28,172	3,080
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Cash equivalents	317	24
Securities owned, at fair value	17,319	8,873
Total	17,636	8,897
Liabilities:
Securities sold, but not yet purchased	582	1,698
Total	582	1,698
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Liabilities:
Deferred consideration (Note 12)	173,024	161,540
Total	173,024	161,540
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Nonrecurring [Member]
Assets:
Thesys Group, Inc. – Series Y preferred stock		3,080
AdvisorEngine – Financial Interests	28,172
Total	$ 28,172	$ 3,080