Collateralized transactions - Narrative (Details)	12 Months Ended
	Dec. 31, 2025 USD ($) customer dealer	Dec. 31, 2024 USD ($)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Bank call loans	$ 76,800,000	$ 252,100,000
Market value of securities	16,000,000.0	69,800,000
Customer securities under customer margin loans that are available to be pledged	2,000,000,000.0
Re-pledged amounts under securities loans agreements	304,100,000
Customer securities pledged directly with the Options Clearing Corporation to secure obligations	336,900,000
Letters of credit outstanding,	0
Amount pledged	$ 1,179,503,000	1,015,604,000
Number of broker-dealers | dealer	4
Receivable from brokers and clearing organizations	$ 117,800,000
Number of customers | customer	2
Receivable from customers	$ 674,900,000
Number of business days for related transactions (in days)	1 day
Accounts Receivable | Customer Concentration Risk | 2 Largest Customers
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Percentage of margin loans	47.80%
Equity Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of securities received as collateral	$ 153,300,000	131,700,000
Equity Securities | Reverse Repurchase Agreements
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of collateral	175,800,000	68,100,000
Equity Securities | 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	$ 49,100,000	39,200,000
Equity Securities | Repurchase Agreements
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of collateral		$ 68,100,000