Fair value measurements - Additional Information (Narrative) (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2022	Dec. 31, 2021
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities owned, at fair value	$ 621,272	$ 634,504	[1]
Valuation adjustment	5,200
Investments	$ 9,150	12,970
Forward or delayed delivery of the underlying instrument with settlement	180 days
Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities owned, at fair value	$ 194,543	51,414
Investments	9,150	12,970
Other Assets | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	4,600
Auction rate securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities owned, at fair value	$ 31,977	31,804
Auction rate securities | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities owned, at fair value		$ 0

[1] Certain prior period reported amounts were reclassified to conform to the current period presentation, see Note 2