Fair value measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2025	Dec. 31, 2024
Securities owned:
Securities owned, at fair value	$ 1,250,802	$ 1,108,206
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	175,712	98,892
Fair Value, Recurring
Assets
Deposits with clearing organizations	27,996	28,071
Securities owned:
Securities owned, at fair value	1,250,802	1,108,206
Investments	32,295	17,983
Trade claims		2,684
Loans	653	432
Derivative contracts:
Derivative contracts, total	34
Total	1,311,780	1,157,376
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	175,712	98,892
Derivative contracts:
Derivative contracts	160	1,071
Total	175,872	99,963
U.S. Treasury securities | Fair Value, Recurring
Securities owned:
Securities owned, at fair value	1,151,564	995,420
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	155,518	82,767
U.S. Agency securities | Fair Value, Recurring
Securities owned:
Securities owned, at fair value	5,925	3,691
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	1	4
Sovereign obligations | Fair Value, Recurring
Securities owned:
Securities owned, at fair value	1,223
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	2,410
Corporate debt and other obligations | Fair Value, Recurring
Securities owned:
Securities owned, at fair value	6,053	9,423
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	1,965	11
Mortgage and other asset-backed securities | Fair Value, Recurring
Securities owned:
Securities owned, at fair value	2,109	8,954
Municipal obligations | Fair Value, Recurring
Securities owned:
Securities owned, at fair value	28,926	34,704
Convertible bonds | Fair Value, Recurring
Securities owned:
Securities owned, at fair value	20,500	21,938
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	6,096	4,998
Corporate equities | Fair Value, Recurring
Securities owned:
Securities owned, at fair value	29,370	23,873
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	9,722	11,112
Money markets | Fair Value, Recurring
Securities owned:
Securities owned, at fair value	5,004	7,551
Auction rate securities | Fair Value, Recurring
Securities owned:
Securities owned, at fair value	128	2,652
Futures | Fair Value, Recurring
Derivative contracts:
Derivative contracts	133	1,071
TBAs | Fair Value, Recurring
Derivative contracts:
Derivative contracts, total	34
Derivative contracts:
Derivative contracts	27
Level 1 | Fair Value, Recurring
Assets
Deposits with clearing organizations	27,996	28,071
Securities owned:
Securities owned, at fair value	1,184,764	1,026,844
Investments	1,600	978
Trade claims		0
Loans	0	0
Derivative contracts:
Derivative contracts, total	$ 0
Derivative Asset, Statement of Financial Position [Extensible Enumeration]	Receivables from brokers, dealers and clearing organizations
Total	$ 1,214,360	1,055,893
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	165,240	93,879
Derivative contracts:
Derivative contracts	$ 133	1,071
Derivative Liability, Statement of Financial Position [Extensible Enumeration]	Payables to brokers, dealers and clearing organizations
Total	$ 165,373	94,950
Level 1 | U.S. Treasury securities | Fair Value, Recurring
Securities owned:
Securities owned, at fair value	1,151,564	995,420
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	155,518	82,767
Level 1 | U.S. Agency securities | Fair Value, Recurring
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 | Fair Value, Recurring
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 | Fair Value, Recurring
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 | Fair Value, Recurring
Securities owned:
Securities owned, at fair value	0	0
Level 1 | Municipal obligations | Fair Value, Recurring
Securities owned:
Securities owned, at fair value	0	0
Level 1 | Convertible bonds | Fair Value, Recurring
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 | Fair Value, Recurring
Securities owned:
Securities owned, at fair value	28,200	23,873
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	9,722	11,112
Level 1 | Money markets | Fair Value, Recurring
Securities owned:
Securities owned, at fair value	5,000	7,551
Level 1 | Auction rate securities | Fair Value, Recurring
Securities owned:
Securities owned, at fair value	0	0
Level 1 | Futures | Fair Value, Recurring
Derivative contracts:
Derivative contracts	133	1,071
Level 1 | TBAs | Fair Value, Recurring
Derivative contracts:
Derivative contracts, total	0
Derivative contracts:
Derivative contracts	0
Level 2 | Fair Value, Recurring
Assets
Deposits with clearing organizations	0	0
Securities owned:
Securities owned, at fair value	62,676	78,710
Investments	13,695	17,005
Trade claims		0
Loans	653	432
Derivative contracts:
Derivative contracts, total	34
Total	77,058	96,147
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	10,472	5,013
Derivative contracts:
Derivative contracts	27	0
Total	10,499	5,013
Level 2 | U.S. Treasury securities | Fair Value, Recurring
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 | Fair Value, Recurring
Securities owned:
Securities owned, at fair value	5,925	3,691
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	1	4
Level 2 | Sovereign obligations | Fair Value, Recurring
Securities owned:
Securities owned, at fair value	1,223
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	2,410
Level 2 | Corporate debt and other obligations | Fair Value, Recurring
Securities owned:
Securities owned, at fair value	3,989	9,423
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	1,965	11
Level 2 | Mortgage and other asset-backed securities | Fair Value, Recurring
Securities owned:
Securities owned, at fair value	2,109	8,954
Level 2 | Municipal obligations | Fair Value, Recurring
Securities owned:
Securities owned, at fair value	28,926	34,704
Level 2 | Convertible bonds | Fair Value, Recurring
Securities owned:
Securities owned, at fair value	20,500	21,938
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	6,096	4,998
Level 2 | Corporate equities | Fair Value, Recurring
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 | Fair Value, Recurring
Securities owned:
Securities owned, at fair value	4	0
Level 2 | Auction rate securities | Fair Value, Recurring
Securities owned:
Securities owned, at fair value	0	0
Level 2 | Futures | Fair Value, Recurring
Derivative contracts:
Derivative contracts	0	0
Level 2 | TBAs | Fair Value, Recurring
Derivative contracts:
Derivative contracts, total	34
Derivative contracts:
Derivative contracts	27
Level 3 | Fair Value, Recurring
Assets
Deposits with clearing organizations	0	0
Securities owned:
Securities owned, at fair value	3,362	2,652
Investments	17,000	0
Trade claims		2,684
Loans	0	0
Derivative contracts:
Derivative contracts, total	0
Total	20,362	5,336
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 | Fair Value, Recurring
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 | Fair Value, Recurring
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 | Fair Value, Recurring
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 | Fair Value, Recurring
Securities owned:
Securities owned, at fair value	2,064	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 | Fair Value, Recurring
Securities owned:
Securities owned, at fair value	0	0
Level 3 | Municipal obligations | Fair Value, Recurring
Securities owned:
Securities owned, at fair value	0	0
Level 3 | Convertible bonds | Fair Value, Recurring
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 | Fair Value, Recurring
Securities owned:
Securities owned, at fair value	1,170	0
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	0	0
Level 3 | Money markets | Fair Value, Recurring
Securities owned:
Securities owned, at fair value	0	0
Level 3 | Auction rate securities
Securities owned:
Securities owned, at fair value	128	2,700
Level 3 | Auction rate securities | Fair Value, Recurring
Securities owned:
Securities owned, at fair value	128	2,652
Level 3 | Futures | Fair Value, Recurring
Derivative contracts:
Derivative contracts	0	$ 0
Level 3 | TBAs | Fair Value, Recurring
Derivative contracts:
Derivative contracts, total	0
Derivative contracts:
Derivative contracts	$ 0