Fair value measurements - Narrative (Details) - USD ($) $ in Thousands	Jun. 30, 2025	Dec. 31, 2024
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities owned, at fair value	$ 1,256,970	$ 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 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities owned, at fair value	163,855	78,710
Equity method investments	6,300	5,900
Level 2 | Auction rate securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities owned, at fair value	0	0
Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities owned, at fair value	3,180	2,652
Convertibles note, fair value	1,900
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