Fair value measurements - Schedule of Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2025	Dec. 31, 2024
ASSETS
Deposits with clearing organizations	$ 27,683	$ 28,071
Securities owned:
Securities owned, at fair value	1,256,970	1,108,206
Investments	17,771	17,983
Trade claim		2,684
Loans	830	432
Total	1,303,354	1,157,376
Securities sold but not yet purchased:
Securities sold but not yet purchased, at fair value	203,341	98,892
Derivative contracts:
Derivative contracts, total	940	1,071
Total	204,281	99,963
Derivative contracts, total	100
U.S. Treasury securities
Securities owned:
Securities owned, at fair value	1,053,781	995,420
Securities sold but not yet purchased:
Securities sold but not yet purchased, at fair value	152,538	82,767
U.S. Agency securities
Securities owned:
Securities owned, at fair value	3,408	3,691
Securities sold but not yet purchased:
Securities sold but not yet purchased, at fair value	1	4
Sovereign obligations
Securities owned:
Securities owned, at fair value	1,230
Securities sold but not yet purchased:
Securities sold but not yet purchased, at fair value	9,159
Corporate debt and other obligations
Securities owned:
Securities owned, at fair value	36,956	9,423
Securities sold but not yet purchased:
Securities sold but not yet purchased, at fair value	14,779	11
Mortgage and other asset-backed securities
Securities owned:
Securities owned, at fair value	7,933	8,954
Municipal obligations
Securities owned:
Securities owned, at fair value	92,617	34,704
Convertible bonds
Securities owned:
Securities owned, at fair value	23,135	21,938
Securities sold but not yet purchased:
Securities sold but not yet purchased, at fair value	16,194	4,998
Corporate equities
Securities owned:
Securities owned, at fair value	30,844	23,873
Securities sold but not yet purchased:
Securities sold but not yet purchased, at fair value	10,670	11,112
Money markets
Securities owned:
Securities owned, at fair value	6,938	7,551
Auction rate securities
Securities owned:
Securities owned, at fair value	128	2,652
Future
Derivative contracts:
Derivative contracts, total	846	1,071
TBAs
Derivative contracts:
Derivative contracts, total	94
Derivative contracts, total	100
Level 1
ASSETS
Deposits with clearing organizations	27,683	28,071
Securities owned:
Securities owned, at fair value	1,089,935	1,026,844
Investments	0	978
Trade claim		0
Loans	0	0
Total	1,117,618	1,055,893
Securities sold but not yet purchased:
Securities sold but not yet purchased, at fair value	163,208	93,879
Derivative contracts:
Derivative contracts, total	846	1,071
Total	164,054	94,950
Derivative contracts, total	0
Level 1 | U.S. Treasury securities
Securities owned:
Securities owned, at fair value	1,053,781	995,420
Securities sold but not yet purchased:
Securities sold but not yet purchased, at fair value	152,538	82,767
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	0
Level 1 | Sovereign 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 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
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	29,654	23,873
Securities sold but not yet purchased:
Securities sold but not yet purchased, at fair value	10,670	11,112
Level 1 | Money markets
Securities owned:
Securities owned, at fair value	6,500	7,551
Level 1 | Auction rate securities
Securities owned:
Securities owned, at fair value	0	0
Level 1 | Future
Derivative contracts:
Derivative contracts, total	846	1,071
Level 1 | TBAs
Derivative contracts:
Derivative contracts, total	0
Derivative contracts, total	0
Level 2
ASSETS
Deposits with clearing organizations	0	0
Securities owned:
Securities owned, at fair value	163,855	78,710
Investments	17,771	17,005
Trade claim		0
Loans	830	432
Total	182,556	96,147
Securities sold but not yet purchased:
Securities sold but not yet purchased, at fair value	40,133	5,013
Derivative contracts:
Derivative contracts, total	94	0
Total	40,227	5,013
Derivative contracts, total	100
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	3,408	3,691
Securities sold but not yet purchased:
Securities sold but not yet purchased, at fair value	1	4
Level 2 | Sovereign obligations
Securities owned:
Securities owned, at fair value	1,230
Securities sold but not yet purchased:
Securities sold but not yet purchased, at fair value	9,159
Level 2 | Corporate debt and other obligations
Securities owned:
Securities owned, at fair value	35,094	9,423
Securities sold but not yet purchased:
Securities sold but not yet purchased, at fair value	14,779	11
Level 2 | Mortgage and other asset-backed securities
Securities owned:
Securities owned, at fair value	7,933	8,954
Level 2 | Municipal obligations
Securities owned:
Securities owned, at fair value	92,617	34,704
Level 2 | Convertible bonds
Securities owned:
Securities owned, at fair value	23,135	21,938
Securities sold but not yet purchased:
Securities sold but not yet purchased, at fair value	16,194	4,998
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	438	0
Level 2 | Auction rate securities
Securities owned:
Securities owned, at fair value	0	0
Level 2 | Future
Derivative contracts:
Derivative contracts, total	0	0
Level 2 | TBAs
Derivative contracts:
Derivative contracts, total	94
Derivative contracts, total	100
Level 3
ASSETS
Deposits with clearing organizations	0	0
Securities owned:
Securities owned, at fair value	3,180	2,652
Investments	0	0
Trade claim		2,684
Loans	0	0
Total	3,180	5,336
Securities sold but not yet purchased:
Securities sold but not yet purchased, at fair value	0	0
Derivative contracts:
Derivative contracts, total	0	0
Total	0	0
Derivative contracts, total	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	0
Level 3 | Sovereign 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 3 | Corporate debt and other obligations
Securities owned:
Securities owned, at fair value	1,862	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
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	1,190	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	128	2,652
Level 3 | Future
Derivative contracts:
Derivative contracts, total	0	$ 0
Level 3 | TBAs
Derivative contracts:
Derivative contracts, total	0
Derivative contracts, total	$ 0