Fair value measurements - Additional Information (Narrative) (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2022	Dec. 31, 2021
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities owned, at fair value	$ 662,677	$ 634,504	[1]
Valuation adjustment	5,200
Investments	$ 8,650	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	$ 82,609	51,414
Investments	8,650	12,970
Other Assets | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	5,800
Auction rate securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities owned, at fair value	$ 33,123	$ 31,804

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