Fair value measurements - Narrative (Details) - USD ($) $ in Thousands	Sep. 30, 2025	Dec. 31, 2024
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities owned, at fair value	$ 1,316,093	$ 1,108,206
Auction rate securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities owned, at fair value	128	2,652
Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities owned, at fair value	3,271	2,652
Convertibles note, fair value	2,000
Warrants at fair value	1,200
Level 3 | Auction rate securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities owned, at fair value	128	2,652
Level 3 | Trade claims
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities owned, at fair value	0	2,700
Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities owned, at fair value	58,268	78,710
Equity method investments	5,900	5,900
Level 2 | Auction rate securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities owned, at fair value	$ 0	$ 0