Collateralized transactions - Narrative (Details)	9 Months Ended
	Sep. 30, 2025 USD ($) dealer customer	Dec. 31, 2024 USD ($)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Bank call loans	$ 262,300,000	$ 252,100,000
Market value of securities	26,900,000	265,900,000
Customer securities under customer margin loans that are available to be pledged	1,900,000,000
Re-pledged amounts under securities loans agreements	233,200,000
Customer securities directly with the Options Clearing Corporation to secure obligations and margin requirements under option contracts written by customers	361,700,000
Outstanding letters of credit	0
Amounts pledged	$ 1,199,263,000	1,015,604,000
Number of broker-dealers | dealer	3
Receivable from brokers and clearing organizations	$ 104,500,000
Number of customers | customer	2
Receivable from customers	$ 652,400,000
Number of business days for related transactions (in days)	1 day
2 Largest Customers | Accounts Receivable | Customer Concentration Risk
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Concentration risk, percentage	48.40%
Corporate equities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of securities received as collateral	$ 160,700,000	131,700,000
Corporate equities | Reverse Repurchase Agreements
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of collateral	238,400,000	68,100,000
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	$ 43,600,000	39,200,000
Corporate equities | Repurchase Agreements
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of collateral		$ 68,100,000