Fair value measurements - Changes in Level 3 Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - Fair Value, Recurring - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024
Trade claims
Assets
Beginning Balance	$ 2,684	$ 0
Total Realized and Unrealized Gains (Losses)	957	0
Purchases and Issuances	534	1,427
Sales and Settlements	(4,175)	0
Transfers In / (Out)	0	1,257
Ending Balance	$ 0	2,684
Fair Value, Asset, Recurring Basis, Unobservable Input Reconciliation, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]	Principal transactions, net
Auction rate securities
Assets
Beginning Balance	$ 2,652	2,713
Total Realized and Unrealized Gains (Losses)	206	9
Purchases and Issuances	0	0
Sales and Settlements	(2,730)	(70)
Transfers In / (Out)	0	0
Ending Balance	$ 128	2,652
Fair Value, Asset, Recurring Basis, Unobservable Input Reconciliation, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]	Principal transactions, net
Private equity securities
Assets
Beginning Balance	$ 0
Total Realized and Unrealized Gains (Losses)	0
Purchases and Issuances	17,000
Sales and Settlements	0
Transfers In / (Out)	0
Ending Balance	17,000	0
Corporate equities
Assets
Beginning Balance	0
Total Realized and Unrealized Gains (Losses)	0
Purchases and Issuances	1,170
Sales and Settlements	0
Transfers In / (Out)	0
Ending Balance	1,170	0
Corporate debt and other obligations
Assets
Beginning Balance	0
Total Realized and Unrealized Gains (Losses)	234
Purchases and Issuances	1,830
Sales and Settlements	0
Transfers In / (Out)	0
Ending Balance	$ 2,064	$ 0