Fair value measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2022	Dec. 31, 2021
ASSETS
Deposits with clearing organizations	$ 41,189	$ 29,083
Securities owned
Securities owned, at fair value	621,272	634,504	[1]
Securities owned and sold, fair value	621,272	634,504
Investments	9,150	12,970
Derivative contracts
Total	671,611	676,649
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	254,652	71,958	[1]
Derivative contracts:
Derivative contracts	374	368
Total	255,026	72,326
Corporate equities
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	10,431	18,679
Money markets
Securities owned
Securities owned, at fair value	314	31
Auction rate securities
Securities owned
Securities owned, at fair value	31,977	31,804
TBAs
Derivative contracts
Derivative contracts, total		92
Derivative contracts:
Derivative contracts	3	81
Futures
Derivative contracts:
Derivative contracts	371	287
U.S. Treasury securities
Securities owned
Securities owned, at fair value	367,425	505,875
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	233,450	42,298
U.S. Agency securities
Securities owned
Securities owned, at fair value	5,538	5,622
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	3	4
Sovereign obligations
Securities owned
Securities owned, at fair value	452	1,494
Corporate debt and other obligations
Securities owned
Securities owned, at fair value	9,382	8,111
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	4,867	2,515
Mortgage and other asset-backed securities
Securities owned
Securities owned, at fair value	2,084	3,889
Municipal obligations
Securities owned
Securities owned, at fair value	162,530	18,520
Convertible bonds
Securities owned
Securities owned, at fair value	14,557	13,778
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	5,900	8,462
Corporate equities | Corporate equities
Securities owned
Securities owned, at fair value	27,013	45,380
Level 1
ASSETS
Deposits with clearing organizations	41,189	29,083
Securities owned
Securities owned, at fair value	394,752	551,286
Derivative contracts
Total	435,941	580,369
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	243,881	60,977
Derivative contracts:
Derivative contracts	371	287
Total	244,252	61,264
Level 1 | Money markets
Securities owned
Securities owned, at fair value	314	31
Level 1 | Futures
Derivative contracts:
Derivative contracts	371	287
Level 1 | U.S. Treasury securities
Securities owned
Securities owned, at fair value	367,425	505,875
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	233,450	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	10,431	18,679
Level 1 | Corporate equities | Corporate equities
Securities owned
Securities owned, at fair value	27,013	45,380
Level 2
Securities owned
Securities owned, at fair value	194,543	51,414
Investments	9,150	12,970
Derivative contracts
Total	203,693	64,476
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	10,771	10,981
Derivative contracts:
Derivative contracts	3	81
Total	10,774	11,062
Level 2 | Auction rate securities
Securities owned
Securities owned, at fair value		0
Level 2 | TBAs
Derivative contracts
Derivative contracts, total		92
Derivative contracts:
Derivative contracts	3	81
Level 2 | U.S. Agency securities
Securities owned
Securities owned, at fair value	5,538	5,622
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	3	4
Level 2 | Sovereign obligations
Securities owned
Securities owned, at fair value	452	1,494
Level 2 | Corporate debt and other obligations
Securities owned
Securities owned, at fair value	9,382	8,111
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	4,867	2,515
Level 2 | Mortgage and other asset-backed securities
Securities owned
Securities owned, at fair value	2,084	3,889
Level 2 | Municipal obligations
Securities owned
Securities owned, at fair value	162,530	18,520
Level 2 | Convertible bonds
Securities owned
Securities owned, at fair value	14,557	13,778
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	5,900	8,462
Level 3
Securities owned
Securities owned, at fair value	31,977	31,804
Investments	0
Derivative contracts
Total	31,977	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	$ 31,977	31,804
Level 3 | Municipal obligations
Securities owned
Securities owned, at fair value		$ 0

[1] Certain prior period reported amounts were reclassified to conform to the current period presentation, see Note 2