Collateralized transactions - Narrative (Details)	6 Months Ended
	Jun. 30, 2024 USD ($) dealer customer	Dec. 31, 2023 USD ($)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Bank call loans	$ 218,800,000	$ 0
Market value of securities	39,100,000	208,000,000
Customer securities under customer margin loans that are available to be pledged	1,600,000,000
Re-pledged amounts under securities loans agreements	186,200,000
Customer securities directly with the Options Clearing Corporation to secure obligations and margin requirements under option contracts written by customers	22,900,000
Outstanding letters of credit	0
Amounts pledged	$ 867,341,000	689,381,000
Number of broker-dealers | dealer	2
Receivable from brokers and clearing organizations	$ 81,200,000
Number of customers | customer	2
Receivable from customers	$ 543,800,000
Number of business days for related transactions (in days)	1 day
Corporate equities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of securities received as collateral	$ 130,300,000	151,900,000
Corporate equities | Reverse Repurchase Agreements
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of collateral	27,500,000	8,800,000
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	$ 50,000,000	61,500,000
Corporate equities | Repurchase Agreements
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of collateral		$ 8,800,000