Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021
ASSETS
Deposits with clearing organizations	$ 38,566	$ 29,083
Securities owned
Securities owned, at fair value	689,046	634,504
Securities owned and sold, fair value	689,046	634,504
Investments	13,934	12,970
Investments	8,405	5,521
Derivative contracts
Derivative contracts, total	6
Total	741,552	676,649
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	97,371	71,958
Derivative contracts:
Derivative contracts	1,858	368
Total	99,229	72,326
Corporate equities
Securities owned
Securities owned, at fair value		45,380
Corporate equities
Securities owned
Securities owned, at fair value	35,254
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	20,429	18,679
Money markets
Securities owned
Securities owned, at fair value		31
Auction rate securities
Securities owned
Securities owned, at fair value	31,804	31,804
U.S. Treasury securities
Securities owned
Securities owned, at fair value	554,315	505,875
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	60,220	42,298
U.S. Agency securities
Securities owned
Securities owned, at fair value	4,643	5,622
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	5	4
Sovereign obligations
Securities owned
Securities owned, at fair value	2,527	1,494
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	4,171
Corporate debt and other obligations
Securities owned
Securities owned, at fair value	14,543	8,111
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	5,805	2,515
Mortgage and other asset-backed securities
Securities owned
Securities owned, at fair value	2,750	3,889
Municipal obligations
Securities owned
Securities owned, at fair value	20,136	18,520
Convertible bonds
Securities owned
Securities owned, at fair value	23,074	13,778
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	6,741	8,462
Level 1
ASSETS
Deposits with clearing organizations	38,566	29,083
Securities owned
Securities owned, at fair value	589,569	551,286
Derivative contracts
Derivative contracts, total	0
Total	628,135	580,369
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	80,649	60,977
Derivative contracts:
Derivative contracts	1,832	287
Total	82,481	61,264
Level 1 | Corporate equities
Securities owned
Securities owned, at fair value	35,254	45,380
Level 1 | Money markets
Securities owned
Securities owned, at fair value		31
Level 1 | U.S. Treasury securities
Securities owned
Securities owned, at fair value	554,315	505,875
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	60,220	42,298
Level 1 | U.S. Agency securities
Securities owned
Securities owned, at fair value	0	0
Level 1 | Corporate debt and other obligations
Securities owned
Securities owned, at fair value	0
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	0
Level 1 | Corporate equities
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	20,429	18,679
Level 2
Securities owned
Securities owned, at fair value	67,673	51,414
Investments	13,934	12,970
Derivative contracts
Derivative contracts, total	6
Total	81,613	64,476
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	16,722	10,981
Derivative contracts:
Derivative contracts	26	81
Total	16,748	11,062
Level 2 | Auction rate securities
Securities owned
Securities owned, at fair value		0
Level 2 | U.S. Agency securities
Securities owned
Securities owned, at fair value	4,643	5,622
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	5	4
Level 2 | Sovereign obligations
Securities owned
Securities owned, at fair value	2,527	1,494
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	4,171
Level 2 | Corporate debt and other obligations
Securities owned
Securities owned, at fair value	14,543	8,111
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	5,805	2,515
Level 2 | Mortgage and other asset-backed securities
Securities owned
Securities owned, at fair value	2,750	3,889
Level 2 | Municipal obligations
Securities owned
Securities owned, at fair value	20,136	18,520
Level 2 | Convertible bonds
Securities owned
Securities owned, at fair value	23,074	13,778
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	6,741	8,462
Level 3
Securities owned
Securities owned, at fair value	31,804	31,804
Investments	0
Derivative contracts
Total	31,804	31,804
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	0
Derivative contracts:
Derivative contracts	0	0
Total	0	0
Level 3 | Auction rate securities
Securities owned
Securities owned, at fair value	31,804	31,804
Level 3 | Municipal obligations
Securities owned
Securities owned, at fair value		0
TBAs
Derivative contracts
Derivative contracts, total	6	92
Derivative contracts:
Derivative contracts	26	81
TBAs | Level 2
Derivative contracts
Derivative contracts, total	6	92
Derivative contracts:
Derivative contracts	26	81
Future
Derivative contracts:
Derivative contracts	1,832	287
Future | Level 1
Derivative contracts:
Derivative contracts	$ 1,832	$ 287