Collateralized transactions - Narrative (Details)	12 Months Ended
	Dec. 31, 2022 USD ($) dealer	Dec. 31, 2021 USD ($)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Bank call loans	$ 0	$ 69,500,000	[1]
Customer securities under customer margin loans that are available to be pledged	1,700,000,000
Re-pledged amounts under securities loans agreements	268,200,000
Customer securities directly with the Options Clearing Corporation to secure obligations and margin requirements under option contracts written by customers	305,900,000
Outstanding letters of credit	0
Amount pledged	$ 175,724,000	266,428,000
Number of broker-dealers | dealer	5
Receivable from brokers and clearing organizations	$ 100,700,000
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	$ 124,100,000	96,400,000
Equity Securities | Reverse Repurchase Agreements
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of collateral	28,000,000	307,300,000
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	$ 39,400,000	29,400,000
Equity Securities | Repurchase Agreements
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of collateral		$ 307,300,000

[1] (1) Certain prior period reported amounts were reclassified to conform to the current period presentation, See Note 2.