Collateralized Transactions - Narrative (Details)	12 Months Ended
	Dec. 31, 2021 USD ($) dealer	Dec. 31, 2020 USD ($)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Bank call loans	$ 69,500,000	$ 82,000,000
Market value of securities	38,500,000
Market value of customer securities	44,400,000
Customer securities under customer margin loans that are available to be pledged	1,800,000,000
Re-pledged amounts under securities loans agreements	205,200,000
Customer securities directly with the Options Clearing Corporation to secure obligations and margin requirements under option contracts written by customers	358,100,000
Outstanding letters of credit	0
Gross Amounts of Recognized Liabilities	306,793,000	430,787,000
Gross Amounts of Recognized Liabilities	244,223,000	249,499,000
Gross amount of recognized liabilities for repurchase agreements and securities loaned	551,016,000
Fair Value of Securities Received as Collateral under Securities Borrowed Transactions	96,400,000
Securities owned, amounts pledged	$ 266,428,000	440,531,000
Number of broker-dealers | dealer	4
Receivable from brokers and clearing organizations	$ 72,900,000
Reverse Repurchase Agreements
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities received as collateral under securities borrowed transaction with market value	307,300,000	88,300,000
Securities Borrowed Transactions
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Customer securities under customer margin loans agreement available to be repledged	29,400,000	36,200,000
Securities received as collateral under securities borrowed transaction with market value		108,000,000
Repurchase Agreements
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Customer securities under customer margin loans agreement available to be repledged	$ 307,300,000	$ 88,300,000
Minimum
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Number of business days for related transactions	1 day
Maximum
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Number of business days for related transactions	2 days