Collateralized Transactions - Additional Information (Narrative) (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2020 USD ($) dealer	Dec. 31, 2019 USD ($)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gross Amounts of Recognized Liabilities	$ 444,290	$ 343,192
Gross Amounts of Recognized Liabilities	291,969	234,343
Bank call loans	156,900	0
Amounts pledged	$ 486,165	357,120
Number of broker-dealers | dealer	3
Receivable from brokers and clearing organizations	$ 75,600
Securities Sold under Agreements to Repurchase, Asset	191,463	55,927
Securities Sold under Agreements to Repurchase, Amount Not Offset Against Collateral	252,827	287,265
Securities Sold under Agreements to Repurchase, Collateral, Right to Reclaim Securities	250,619	285,264
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	2,208	2,001
Securities Loaned, Asset	0	0
Securities Loaned, Amount Not Offset Against Collateral	291,969	234,343
Securities Loaned, Collateral, Right to Reclaim Securities	284,998	228,548
Securities Loaned, Collateral, Right to Reclaim Cash	0	0
Securities Loaned, Offset Against Collateral, Net of Not Subject to Master Netting Arrangement, Policy Election	6,971	5,795
Derivative Liability, Securities Sold under Agreements to Repurchase, Securities Loaned, Gross	736,259	577,535
Derivative Liability, Securities Sold under Agreements to Repurchase, Securities Loaned, Asset	191,463	55,927
Derivative Liability, Securities Sold under Agreements to Resell, Securities Loaned, Amount Not Offset Against Collateral	544,796	521,608
Derivative Liability, Securities Sold under Agreements to Repurchase, Securities Loaned, Collateral, Right to Reclaim Securities	535,617	513,812
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	9,179	7,796
Firm's Securities Fair Value, Collateral for Bank Loans	44,900
Customer securities directly with the Options Clearing Corporation to secure obligations and margin requirements under option contracts written by customers	382,800
Customer Securities under Customer Margin Loans that are Available to be Pledged, Re-pledged Amount	247,300
Fair Value of Securities Received as Collateral under Securities Borrowed Transactions	125,800	96,300
Fair Value of Securities Received as Collateral under Reverse Repurchase agreements	191,400	55,800
Fair value of Securities Received as Collateral, Sold and Re-Pledged under Securities Loaned	36,700	19,300
Fair value of Securities Received as Collateral, Sold and Re-Pledged under Repurchase Agreements	191,400	$ 55,800
Customer Securities Fair Value, Collateral for Bank Loans	140,600
customer securities under customer margin loans that are available to be pledged	1,500,000
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	$ 736,259
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