Fair value measurements - Schedule of Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2026	Dec. 31, 2025
Assets
Deposits with clearing organizations	$ 28,237	$ 27,996
Securities owned:
Securities owned, at fair value	1,265,918	1,250,802
Investments	29,099	32,295
Loans	296	653
Derivative contracts
Derivative contracts, total	98	34
Total	1,323,648	1,311,780
Securities sold but not yet purchased:
Securities sold but not yet purchased, at fair value	231,804	175,712
Derivative contracts
Derivative contracts, total	3,904	160
Total	235,708	175,872
U.S. Treasury securities
Securities owned:
Securities owned, at fair value	1,160,684	1,151,564
Securities sold but not yet purchased:
Securities sold but not yet purchased, at fair value	209,798	155,518
U.S. Agency securities
Securities owned:
Securities owned, at fair value	5,878	5,925
Securities sold but not yet purchased:
Securities sold but not yet purchased, at fair value	2	1
Sovereign obligations
Securities owned:
Securities owned, at fair value	5,316	1,223
Securities sold but not yet purchased:
Securities sold but not yet purchased, at fair value	997	2,410
Corporate debt and other obligations
Securities owned:
Securities owned, at fair value	10,143	6,053
Securities sold but not yet purchased:
Securities sold but not yet purchased, at fair value	1,275	1,965
Mortgage and other asset-backed securities
Securities owned:
Securities owned, at fair value	2,140	2,109
Municipal obligations
Securities owned:
Securities owned, at fair value	28,350	28,926
Convertible bonds
Securities owned:
Securities owned, at fair value	17,826	20,500
Securities sold but not yet purchased:
Securities sold but not yet purchased, at fair value	10,652	6,096
Corporate equities
Securities owned:
Securities owned, at fair value	30,223	29,370
Securities sold but not yet purchased:
Securities sold but not yet purchased, at fair value	9,080	9,722
Money markets
Securities owned:
Securities owned, at fair value	5,230	5,004
Other debt securities
Securities owned:
Securities owned, at fair value	128	128
Futures
Derivative contracts
Derivative contracts, total	3,812	133
TBAs
Derivative contracts
Derivative contracts, total	98	34
Derivative contracts
Derivative contracts, total	92	27
Level 1
Assets
Deposits with clearing organizations	28,237	27,996
Securities owned:
Securities owned, at fair value	1,194,737	1,184,764
Investments	1,600	1,600
Loans	0	0
Derivative contracts
Derivative contracts, total	0	0
Total	1,224,574	1,214,360
Securities sold but not yet purchased:
Securities sold but not yet purchased, at fair value	218,878	165,240
Derivative contracts
Derivative contracts, total	3,812	133
Total	222,690	165,373
Level 1 | U.S. Treasury securities
Securities owned:
Securities owned, at fair value	1,160,684	1,151,564
Securities sold but not yet purchased:
Securities sold but not yet purchased, at fair value	209,798	155,518
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	0
Securities sold but not yet purchased:
Securities sold but not yet purchased, at fair value	0	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	29,053	28,200
Securities sold but not yet purchased:
Securities sold but not yet purchased, at fair value	9,080	9,722
Level 1 | Money markets
Securities owned:
Securities owned, at fair value	5,000	5,000
Level 1 | Other debt securities
Securities owned:
Securities owned, at fair value	0	0
Level 1 | Futures
Derivative contracts
Derivative contracts, total	3,812	133
Level 1 | TBAs
Derivative contracts
Derivative contracts, total	0	0
Derivative contracts
Derivative contracts, total	0	0
Level 2
Assets
Deposits with clearing organizations	0	0
Securities owned:
Securities owned, at fair value	67,730	62,676
Investments	10,499	13,695
Loans	296	653
Derivative contracts
Derivative contracts, total	98	34
Total	78,623	77,058
Securities sold but not yet purchased:
Securities sold but not yet purchased, at fair value	12,926	10,472
Derivative contracts
Derivative contracts, total	92	27
Total	13,018	10,499
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	5,878	5,925
Securities sold but not yet purchased:
Securities sold but not yet purchased, at fair value	2	1
Level 2 | Sovereign obligations
Securities owned:
Securities owned, at fair value	5,316	1,223
Securities sold but not yet purchased:
Securities sold but not yet purchased, at fair value	997	2,410
Level 2 | Corporate debt and other obligations
Securities owned:
Securities owned, at fair value	7,990	3,989
Securities sold but not yet purchased:
Securities sold but not yet purchased, at fair value	1,275	1,965
Level 2 | Mortgage and other asset-backed securities
Securities owned:
Securities owned, at fair value	2,140	2,109
Level 2 | Municipal obligations
Securities owned:
Securities owned, at fair value	28,350	28,926
Level 2 | Convertible bonds
Securities owned:
Securities owned, at fair value	17,826	20,500
Securities sold but not yet purchased:
Securities sold but not yet purchased, at fair value	10,652	6,096
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	230	4
Level 2 | Other debt securities
Securities owned:
Securities owned, at fair value	0	0
Level 2 | Futures
Derivative contracts
Derivative contracts, total	0	0
Level 2 | TBAs
Derivative contracts
Derivative contracts, total	98	34
Derivative contracts
Derivative contracts, total	92	27
Level 3
Assets
Deposits with clearing organizations	0	0
Securities owned:
Securities owned, at fair value	3,451	3,362
Investments	17,000	17,000
Loans	0	0
Derivative contracts
Derivative contracts, total	0	0
Total	20,451	20,362
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
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	0
Securities sold but not yet purchased:
Securities sold but not yet purchased, at fair value	0	0
Level 3 | Corporate debt and other obligations
Securities owned:
Securities owned, at fair value	2,153	2,064
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	1,170	1,170
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 | Other debt securities
Securities owned:
Securities owned, at fair value	128	128
Level 3 | Futures
Derivative contracts
Derivative contracts, total	0	0
Level 3 | TBAs
Derivative contracts
Derivative contracts, total	0	0
Derivative contracts
Derivative contracts, total	$ 0	$ 0