Fair value measurements - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities owned, at fair value	$ 1,250,802,000	$ 1,108,206,000
Forward or delayed delivery of the underlying instrument with settlement	180 days
Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Equity method investment	$ 6,300,000	5,900,000
Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
convertible note, fair value	2,100,000	0
Warrants at fair value	1,200,000	0
Level 3 | Auction rate securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities owned, at fair value	128,000	2,700,000
Level 3 | Trade claims
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities owned, at fair value	0	$ 2,700,000
Level 3 | Private Equity Funds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Equity security	$ 17,000,000.0