Fair value measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2024	Dec. 31, 2023
ASSETS
Deposits with clearing organizations	$ 28,071	$ 34,789
Securities owned
Securities owned, at fair value	1,108,206	795,312
Investments	17,983	18,785
Trade claims	2,684
Loans	432
Securities purchased under agreements to resell	0	5,842
Derivative contracts:
Derivative contracts, total		13
Total	1,157,376	854,741
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	98,892	31,676
Derivative contracts:
Derivative contracts	$ 1,071	737
Derivative Liability, Statement of Financial Position [Extensible Enumeration]	Receivable from brokers, dealers and clearing organizations
Total	$ 99,963	32,413
U.S. Treasury securities
Securities owned
Securities owned, at fair value	995,420	695,346
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	82,767	14,603
U.S. Agency securities
Securities owned
Securities owned, at fair value	3,691	2
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	4
Corporate debt and other obligations
Securities owned
Securities owned, at fair value	9,423	5,769
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	11	1,508
Mortgage and other asset-backed securities
Securities owned
Securities owned, at fair value	8,954	6,627
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value		2
Municipal obligations
Securities owned
Securities owned, at fair value	34,704	35,333
Convertible bonds
Securities owned
Securities owned, at fair value	21,938	16,735
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	4,998	2,136
Corporate equities
Securities owned
Securities owned, at fair value	23,873	27,170
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	11,112	13,427
Money markets
Securities owned
Securities owned, at fair value	7,551	5,617
Auction rate securities
Securities owned
Securities owned, at fair value	2,652	2,713
Futures
Derivative contracts:
Derivative contracts, total		2
Derivative contracts:
Derivative contracts	1,071	735
TBAs
Derivative contracts:
Derivative contracts, total		11
Derivative contracts:
Derivative contracts		2
Level 1
ASSETS
Deposits with clearing organizations	28,071	34,789
Securities owned
Securities owned, at fair value	1,026,844	727,916
Investments	978	1,872
Trade claims
Loans
Securities purchased under agreements to resell		0
Derivative contracts:
Derivative contracts, total		2
Total	1,055,893	764,579
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	93,879	28,030
Derivative contracts:
Derivative contracts	1,071	735
Total	94,950	28,765
Level 1 | U.S. Treasury securities
Securities owned
Securities owned, at fair value	995,420	695,346
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	82,767	14,603
Level 1 | U.S. Agency securities
Securities owned
Securities owned, at fair value	0	0
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	0
Level 1 | Corporate debt and other obligations
Securities owned
Securities owned, at fair value	0	0
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	0	0
Level 1 | Mortgage and other asset-backed securities
Securities owned
Securities owned, at fair value	0	0
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value		0
Level 1 | Municipal obligations
Securities owned
Securities owned, at fair value	0	0
Level 1 | Convertible bonds
Securities owned
Securities owned, at fair value	0	0
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	0	0
Level 1 | Corporate equities
Securities owned
Securities owned, at fair value	23,873	27,170
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	11,112	13,427
Level 1 | Money markets
Securities owned
Securities owned, at fair value	7,551	5,400
Level 1 | Auction rate securities
Securities owned
Securities owned, at fair value	0	0
Level 1 | Futures
Derivative contracts:
Derivative contracts, total		2
Derivative contracts:
Derivative contracts	1,071	735
Level 1 | TBAs
Derivative contracts:
Derivative contracts, total		0
Derivative contracts:
Derivative contracts		0
Level 2
ASSETS
Deposits with clearing organizations	0	0
Securities owned
Securities owned, at fair value	78,710	64,683
Investments	17,005	16,913
Trade claims
Loans	432
Securities purchased under agreements to resell		5,842
Derivative contracts:
Derivative contracts, total		11
Total	96,147	87,449
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	5,013	3,646
Derivative contracts:
Derivative contracts	0	2
Total	5,013	3,648
Level 2 | U.S. Treasury securities
Securities owned
Securities owned, at fair value	0	0
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	0	0
Level 2 | U.S. Agency securities
Securities owned
Securities owned, at fair value	3,691	2
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	4
Level 2 | Corporate debt and other obligations
Securities owned
Securities owned, at fair value	9,423	5,769
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	11	1,508
Level 2 | Mortgage and other asset-backed securities
Securities owned
Securities owned, at fair value	8,954	6,627
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value		2
Level 2 | Municipal obligations
Securities owned
Securities owned, at fair value	34,704	35,333
Level 2 | Convertible bonds
Securities owned
Securities owned, at fair value	21,938	16,735
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	4,998	2,136
Level 2 | Corporate equities
Securities owned
Securities owned, at fair value	0	0
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	0	0
Level 2 | Money markets
Securities owned
Securities owned, at fair value	0	217
Level 2 | Auction rate securities
Securities owned
Securities owned, at fair value	0	0
Level 2 | Futures
Derivative contracts:
Derivative contracts, total		0
Derivative contracts:
Derivative contracts	0	0
Level 2 | TBAs
Derivative contracts:
Derivative contracts, total		11
Derivative contracts:
Derivative contracts		2
Level 3
ASSETS
Deposits with clearing organizations	0	0
Securities owned
Securities owned, at fair value	2,652	2,713
Investments	0	0
Trade claims	2,684
Loans
Securities purchased under agreements to resell
Derivative contracts:
Derivative contracts, total		0
Total	5,336	2,713
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	0	0
Derivative contracts:
Derivative contracts	0	0
Total	0	0
Level 3 | U.S. Treasury securities
Securities owned
Securities owned, at fair value	0	0
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	0	0
Level 3 | U.S. Agency securities
Securities owned
Securities owned, at fair value	0	0
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	0
Level 3 | Corporate debt and other obligations
Securities owned
Securities owned, at fair value	0	0
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	0	0
Level 3 | Mortgage and other asset-backed securities
Securities owned
Securities owned, at fair value	0	0
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value		0
Level 3 | Municipal obligations
Securities owned
Securities owned, at fair value	0	0
Level 3 | Convertible bonds
Securities owned
Securities owned, at fair value	0	0
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	0	0
Level 3 | Corporate equities
Securities owned
Securities owned, at fair value	0	0
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	0	0
Level 3 | Money markets
Securities owned
Securities owned, at fair value	0	0
Level 3 | Auction rate securities
Securities owned
Securities owned, at fair value	2,652	2,713
Level 3 | Futures
Derivative contracts:
Derivative contracts, total		0
Derivative contracts:
Derivative contracts	$ 0	0
Level 3 | TBAs
Derivative contracts:
Derivative contracts, total		0
Derivative contracts:
Derivative contracts		$ 0