Collateralized Transactions - Additional Information (Narrative) (Details)		12 Months Ended
		Dec. 31, 2020 USD ($) dealer	Dec. 31, 2019 USD ($)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Bank call loans		$ 82,000,000	$ 0
Outstanding letters of credit		0
Gross Amounts of Recognized Liabilities			343,192,000
Gross Amounts of Recognized Liabilities	[1]		234,343,000
Amounts pledged		$ 440,531,000	357,120,000
Number of broker-dealers | dealer		3
Receivable from brokers and clearing organizations		$ 69,700,000
Firm's Securities Fair Value, Collateral for Bank Loans		63,700,000
Customer Securities Fair Value, Collateral for Bank Loans		28,400,000
customer securities under customer margin loans that are available to be pledged		1,600,000,000
Customer Securities under Customer Margin Loans that are Available to be Pledged, Re-pledged Amount		208,000,000.0
Customer securities directly with the Options Clearing Corporation to secure obligations and margin requirements under option contracts written by customers		377,900,000
Fair Value of Securities Received as Collateral under Securities Borrowed Transactions		108,000,000.0
Reverse Repurchase Agreements
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities received as collateral under securities borrowed transaction with market value			55,800,000
Securities Borrowed Transactions
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Customer securities under customer margin loans agreement available to be repledged			19,300,000
Securities received as collateral under securities borrowed transaction with market value			96,300,000
Repurchase Agreements [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Customer securities under customer margin loans agreement available to be repledged			$ 55,800,000
Maturity Overnight and on Demand [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities Loaned and Securities Sold under Agreement to Repurchase, Gross Including Not Subject to Master Netting Arrangement		680,286,000
U.S. treasury, agency and sovereign obligations | Maturity Overnight and on Demand [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gross Amounts of Recognized Liabilities		430,787,000
Equity Securities [Member] | Maturity Overnight and on Demand [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gross Amounts of Recognized Liabilities		$ 249,499,000
Minimum [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Number of business days for related transactions		1 day
Maximum [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Number of business days for related transactions		2 days

[1] (2) Included in payable to brokers, dealers and clearing organizations on the consolidated balance sheet.