Collateralized transactions - Narrative (Details)	3 Months Ended
	Mar. 31, 2026 USD ($) dealer customer	Dec. 31, 2025 USD ($)	Dec. 31, 2024 USD ($)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Bank call loans	$ 287,900,000	$ 76,800,000
Market value of securities	27,400,000		$ 293,300,000
Customer securities under customer margin loans that are available to be pledged	2,000,000,000.0
Re-pledged amounts under securities loans agreements	275,400,000
Customer securities directly with the Options Clearing Corporation to secure obligations and margin requirements under option contracts written by customers	442,800,000
Outstanding letters of credit	0
Amounts pledged	$ 1,172,973,000	1,179,503,000
Number of broker-dealers | dealer	2
Receivable from brokers and clearing organizations	$ 60,400,000
Number of customers | customer	2
Receivable from customers	$ 658,200,000
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	45.20%
Corporate equities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of securities received as collateral	$ 158,900,000	153,300,000
Corporate equities | Reverse Repurchase Agreements
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of collateral	193,900,000	175,800,000
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	$ 43,700,000	49,100,000
Corporate equities | Repurchase Agreements
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of collateral		$ 175,800,000