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Collateralized Transactions - Additional Information (Narrative) (Details)
 6 Months Ended
Jun. 30, 2021
USD ($)
dealer
Dec. 31, 2020
USD ($)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Gross Amounts of Recognized Liabilities $ 373,472,000 $ 430,787,000
Gross Amounts of Recognized Liabilities 267,521,000 249,499,000
Bank call loans 79,600,000 82,000,000
Amounts pledged $ 305,822,000 440,531,000
Number of broker-dealers | dealer 3  
Receivable from brokers and clearing organizations $ 80,300,000  
Securities Sold under Agreements to Repurchase, Asset 141,871,000 88,349,000
Securities Sold under Agreements to Repurchase, Amount Not Offset Against Collateral 231,601,000 342,438,000
Securities Sold under Agreements to Repurchase, Collateral, Right to Reclaim Securities 230,659,000 340,632,000
Securities Sold under Agreements to Repurchase, Collateral, Right to Reclaim Cash 0 0
Securities Sold under Agreements to Repurchase, Offset Against Collateral, Net of Not Subject to Master Netting Arrangement, Policy Election 942,000 1,806,000
Securities Loaned, Asset 0 0
Securities Loaned, Amount Not Offset Against Collateral 267,521,000 249,499,000
Securities Loaned, Collateral, Right to Reclaim Securities 260,807,000 242,318,000
Securities Loaned, Collateral, Right to Reclaim Cash 0 0
Securities Loaned, Offset Against Collateral, Net of Not Subject to Master Netting Arrangement, Policy Election 6,714,000 7,181,000
Derivative Liability, Securities Sold under Agreements to Repurchase, Securities Loaned, Gross 640,993,000 680,286,000
Derivative Liability, Securities Sold under Agreements to Repurchase, Securities Loaned, Asset 141,871,000 88,349,000
Derivative Liability, Securities Sold under Agreements to Resell, Securities Loaned, Amount Not Offset Against Collateral 499,122,000 591,937,000
Derivative Liability, Securities Sold under Agreements to Repurchase, Securities Loaned, Collateral, Right to Reclaim Securities 491,466,000 582,950,000
Derivative Liability, Securities Sold under Agreements to Repurchase, Securities Loaned, Collateral, Right to Reclaim Cash 0 0
Derivative Liability, Securities Sold under Agreements to Repurchase, Securities Loaned, Offset Against Collateral, Net of Not Subject to Master Netting Arrangement, Policy Election 7,656,000 8,987,000
Firm's Securities Fair Value, Collateral for Bank Loans 43,800,000  
Customer securities directly with the Options Clearing Corporation to secure obligations and margin requirements under option contracts written by customers 273,200,000  
Customer Securities under Customer Margin Loans that are Available to be Pledged, Re-pledged Amount 222,100,000  
Customer Securities Fair Value, Collateral for Bank Loans 50,600,000  
customer securities under customer margin loans that are available to be pledged 1,700,000,000  
Securities Purchased under Agreements to Resell, Liability (141,871,000) (88,349,000)
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 640,993,000  
Equity Securities [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair Value of Securities Received as Collateral under Securities Borrowed Transactions 116,000,000.0 108,000,000.0
Fair value of Securities Received as Collateral, Sold and Re-Pledged under Securities Loaned 38,800,000 36,200,000
Securities Sold under Agreements to Repurchase, Fair Value of Collateral $ 372,500,000 $ 88,300,000
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