Collateralized transactions - Narrative (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2022 USD ($) dealer	Dec. 31, 2021 USD ($)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Bank call loans	$ 78,150	$ 69,500
Market value of securities	26,000
Market value of customer securities	65,300
Customer securities under customer margin loans that are available to be pledged	1,800,000
Re-pledged amounts under securities loans agreements	252,200
Customer securities directly with the Options Clearing Corporation to secure obligations and margin requirements under option contracts written by customers	424,000
Outstanding letters of credit	0
Amounts pledged	$ 551,736	266,428
Number of broker-dealers | dealer	3
Receivable from brokers and clearing organizations	$ 66,200
Corporate equities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of securities received as collateral	94,500	96,400
Corporate equities | Reverse Repurchase Agreements
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of collateral	44,200	307,300
Corporate equities | Securities Borrowed Transactions
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of Securities Received as Collateral, Sold and Re-Pledged under Securities Loaned	$ 31,400	29,400
Corporate equities | Repurchase Agreements
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of collateral		$ 307,300
Minimum
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Number of business days for related transactions	1 day
Maximum
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Number of business days for related transactions	2 days