Collateralized transactions - Narrative (Details)	12 Months Ended
	Dec. 31, 2023 USD ($) dealer	Dec. 31, 2022 USD ($)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Bank call loans	$ 0	$ 0
Customer securities under customer margin loans that are available to be pledged	1,600,000,000
Re-pledged amounts under securities loans agreements	211,300,000
Customer securities pledged directly with the Options Clearing Corporation to secure obligations	129,200,000
Letters of credit outstanding,	0
Amount pledged	$ 689,381,000	175,724,000
Number of broker-dealers | dealer	3
Receivable from brokers and clearing organizations	$ 107,600,000
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
Equity Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of securities received as collateral	$ 151,900,000	124,100,000
Equity Securities | Reverse Repurchase Agreements
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of collateral	8,800,000	28,000,000
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	$ 61,500,000	39,400,000
Equity Securities | Repurchase Agreements
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of collateral		$ 28,000,000