Fair Value Measurements - Summary of Categorization of Assets and Liabilities Measured at Fair Value (Detail) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021
Assets:
Securities owned, at fair value	$ 133,846	$ 127,166
Liabilities:
Deferred consideration (Note 9)	245,177	228,062
Fair Value, Measurements, Recurring [Member]
Assets:
Cash equivalents	449	11,488
Total	140,995	138,654
Liabilities:
Deferred consideration (Note 9)	245,177	228,062
Fair Value, Measurements, Recurring [Member] | ETFs [Member]
Assets:
Securities owned, at fair value	16,035	18,812
Fair Value, Measurements, Recurring [Member] | Pass-through GSEs [Member]
Assets:
Securities owned, at fair value	115,858	106,245
Fair Value, Measurements, Recurring [Member] | Corporate Bonds [Member]
Assets:
Securities owned, at fair value	1,953	2,109
Securrency - Series A convertible preferred stock | Fair Value, Measurements, Nonrecurring [Member]
Assets:
Investment owned, at fair value		8,488
Fnality Convertible Note Member [Member] | Fair Value, Measurements, Recurring [Member]
Investments
Fnality International Limited – convertible note (Note 7)	6,700
Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Cash equivalents	449	11,488
Total	40,987	55,020
Liabilities:
Deferred consideration (Note 9)	0	0
Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | ETFs [Member]
Assets:
Securities owned, at fair value	16,035	18,812
Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Pass-through GSEs [Member]
Assets:
Securities owned, at fair value	24,503	24,720
Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Corporate Bonds [Member]
Assets:
Securities owned, at fair value	0	0
Level 1 [Member] | Securrency - Series A convertible preferred stock | Fair Value, Measurements, Nonrecurring [Member]
Assets:
Investment owned, at fair value		0
Level 1 [Member] | Fnality Convertible Note Member [Member] | Fair Value, Measurements, Recurring [Member]
Investments
Fnality International Limited – convertible note (Note 7)	0
Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Cash equivalents	0	0
Total	93,308	83,634
Liabilities:
Deferred consideration (Note 9)	0	0
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | ETFs [Member]
Assets:
Securities owned, at fair value	0	0
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Pass-through GSEs [Member]
Assets:
Securities owned, at fair value	91,355	81,525
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Corporate Bonds [Member]
Assets:
Securities owned, at fair value	1,953	2,109
Level 2 [Member] | Securrency - Series A convertible preferred stock | Fair Value, Measurements, Nonrecurring [Member]
Assets:
Investment owned, at fair value		0
Level 2 [Member] | Fnality Convertible Note Member [Member] | Fair Value, Measurements, Recurring [Member]
Investments
Fnality International Limited – convertible note (Note 7)	0
Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Cash equivalents	0	0
Total	6,700	0
Liabilities:
Deferred consideration (Note 9)	245,177	228,062
Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | ETFs [Member]
Assets:
Securities owned, at fair value	0	0
Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Pass-through GSEs [Member]
Assets:
Securities owned, at fair value	0	0
Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Corporate Bonds [Member]
Assets:
Securities owned, at fair value	0	0
Level 3 [Member] | Securrency - Series A convertible preferred stock | Fair Value, Measurements, Nonrecurring [Member]
Assets:
Investment owned, at fair value		$ 8,488
Level 3 [Member] | Fnality Convertible Note Member [Member] | Fair Value, Measurements, Recurring [Member]
Investments
Fnality International Limited – convertible note (Note 7)	$ 6,700