Fair Value Measurements - Summary of Categorization of Assets and Liabilities Measured at Fair Value (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Liabilities:
Deferred consideration (Note 11)	$ 175,300	$ 173,024
Fair Value, Measurements, Recurring [Member]
Assets:
Cash equivalents	3,885	317
Securities owned, at fair value	20,261	17,319
Total	24,146	17,636
Liabilities:
Deferred consideration (Note 11)	175,300	173,024
Securities sold, but not yet purchased	469	582
Total	175,769	173,606
Fair Value, Measurements, Nonrecurring [Member]
Assets:
AdvisorEngine – Financial Interests	8,500	28,172
Total	8,500	28,172
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Cash equivalents	3,885	317
Securities owned, at fair value	20,261	17,319
Total	24,146	17,636
Liabilities:
Securities sold, but not yet purchased	469	582
Total	469	582
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Liabilities:
Deferred consideration (Note 11)	175,300	173,024
Total	175,300	173,024
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Nonrecurring [Member]
Assets:
AdvisorEngine – Financial Interests	8,500	28,172
Total	$ 8,500	$ 28,172