Fair Value Measurements - Summary of Categorization of Assets and Liabilities Measured at Fair Value (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Mar. 08, 2021	Dec. 31, 2020	Aug. 13, 2020	Jun. 16, 2020
Assets:
Securities owned, at fair value	$ 34,771		$ 34,895
Liabilities:
Deferred consideration (Note 12)	227,146		230,137
Fair Value, Measurements, Recurring [Member]
Assets:
Cash equivalents	502		660
Total	35,273		35,555
Liabilities:
Deferred consideration (Note 12)	227,146		230,137
Fair Value, Measurements, Recurring [Member] | Exchange Traded Funds [Member]
Assets:
Securities owned, at fair value	23,862		24,165
Fair Value, Measurements, Recurring [Member] | Federal Agency [Member]
Assets:
Securities owned, at fair value	8,832		8,613
Fair Value, Measurements, Recurring [Member] | All Other Corporate Bonds [Member]
Assets:
Securities owned, at fair value	2,077		2,117
Fair Value, Measurements, Nonrecurring [Member]
Assets:
AdvisorEngine Inc. – Financial interests			0
Investment owned, at fair value			0
Total			0
Liabilities:
Convertible notes			170,191	$ 24,344	$ 145,847
Fair Value, Measurements, Nonrecurring [Member] | Series A convertible preferred stock [Member]
Assets:
Investment owned, at fair value	8,349	$ 8,349
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Cash equivalents	502		660
Total	24,364		24,825
Liabilities:
Deferred consideration (Note 12)			0
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Exchange Traded Funds [Member]
Assets:
Securities owned, at fair value	23,862		24,165
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Nonrecurring [Member]
Assets:
AdvisorEngine Inc. – Financial interests			0
Investment owned, at fair value			0
Total			0
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Total	10,909		10,730
Liabilities:
Deferred consideration (Note 12)			0
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Federal Agency [Member]
Assets:
Securities owned, at fair value	8,832		8,613
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | All Other Corporate Bonds [Member]
Assets:
Securities owned, at fair value	2,077		2,117
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Nonrecurring [Member]
Assets:
AdvisorEngine Inc. – Financial interests			0
Investment owned, at fair value			0
Total			0
Liabilities:
Convertible notes			170,191
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Cash equivalents			0
Total			0
Liabilities:
Deferred consideration (Note 12)	227,146		230,137
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Nonrecurring [Member]
Assets:
AdvisorEngine Inc. – Financial interests			0
Investment owned, at fair value			0
Total			$ 0
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Nonrecurring [Member] | Series A convertible preferred stock [Member]
Assets:
Investment owned, at fair value	$ 8,349