Collateralized transactions - Narrative (Details)	3 Months Ended
Mar. 31, 2024 USD ($) dealer	Dec. 31, 2023 USD ($)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Bank call loans	$ 94,350,000	$ 0
Market value of securities	24,700,000	82,300,000
Customer securities under customer margin loans that are available to be pledged	1,700,000,000
Re-pledged amounts under securities loans agreements	229,000,000
Customer securities directly with the Options Clearing Corporation to secure obligations and margin requirements under option contracts written by customers	32,000,000
Outstanding letters of credit	0
Amounts pledged	$ 917,804,000	689,381,000
Number of broker-dealers \| dealer	3
Receivable from brokers and clearing organizations	$ 97,900,000
Minimum
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Number of business days for related transactions (in days)	1 day
Maximum
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Number of business days for related transactions (in days)	2 days
Corporate equities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of securities received as collateral	$ 142,500,000	151,900,000
Corporate equities \| Reverse Repurchase Agreements
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of collateral	578,400,000	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	$ 56,700,000	61,500,000
Corporate equities \| Repurchase Agreements
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of collateral		$ 8,800,000

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- Definition

Customer Securities Pledged with the Options Clearing Corporation

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No definition available.

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Fair value of Securities Received as Collateral, Sold and Re-Pledged under Securities Loaned

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Pledged securities owned and pledged as collateral amount eligible to be repledged by counterparty carrying value.

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- Definition

Receivables From Brokers-Dealers And Clearing Organizations, Number Of Major Broker-Dealers

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- Definition

Amount of short-term bank loan secured by broker-dealer customer's security.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name Rule 15c3-1
-Number 240
-Section 15c3-1
-Publisher SEC

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

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Fair value of securities pledged as collateral against securities sold under agreement to repurchase.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 210
-SubTopic 20
-Name Accounting Standards Codification
-Section 55
-Paragraph 12
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147483444/210-20-55-12

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