Collateralized Transactions - Additional Information (Narrative) (Details)	3 Months Ended
	Mar. 31, 2020 USD ($) dealer	Dec. 31, 2019 USD ($)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gross Amounts of Recognized Liabilities	$ 135,870,000	$ 343,192,000
Gross Amounts of Recognized Liabilities	181,745,000	234,343,000
Bank call loans	203,100,000	0
Customer Securities for which Entity has Right to Sell or Repledge, Fair Value	1,300,000,000
Customer securities under customer margin loans agreement available to be repledged	149,400,000
Outstanding letters of credit	0
Amounts pledged	150,547,000	357,120,000
Carrying value of securities owned by the Company loaned or pledged	$ 31,700,000	171,400,000
Number of broker-dealers | dealer	4
Receivable from brokers and clearing organizations	$ 80,200,000
Securities Sold under Agreements to Repurchase, Asset	11,767,000	55,927,000
Securities Sold under Agreements to Repurchase, Amount Not Offset Against Collateral	124,103,000	287,265,000
Securities Sold under Agreements to Repurchase, Collateral, Right to Reclaim Securities	123,506,000	285,264,000
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	597,000	2,001,000
Securities Loaned, Asset	0	0
Securities Loaned, Amount Not Offset Against Collateral	181,745,000	234,343,000
Securities Loaned, Collateral, Right to Reclaim Securities	174,038,000	228,548,000
Securities Loaned, Collateral, Right to Reclaim Cash	0	0
Securities Loaned, Offset Against Collateral, Net of Not Subject to Master Netting Arrangement, Policy Election	7,707,000	5,795,000
Derivative Liability, Securities Sold under Agreements to Repurchase, Securities Loaned, Gross	317,615,000	577,535,000
Derivative Liability, Securities Sold under Agreements to Repurchase, Securities Loaned, Asset	11,767,000	55,927,000
Derivative Liability, Securities Sold under Agreements to Resell, Securities Loaned, Amount Not Offset Against Collateral	305,848,000	521,608,000
Derivative Liability, Securities Sold under Agreements to Repurchase, Securities Loaned, Collateral, Right to Reclaim Securities	297,544,000	513,812,000
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	8,304,000	7,796,000
Reverse Repurchase Agreements
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities received as collateral under securities borrowed transaction with market value	11,800,000	55,800,000
Securities Loaned [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Customer securities under customer margin loans agreement available to be repledged	22,200,000	19,300,000
Repurchase Agreements [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Customer securities under customer margin loans agreement available to be repledged	11,800,000	55,800,000
Securities Borrowed Transactions
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities received as collateral under securities borrowed transaction with market value	98,900,000	$ 96,300,000
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	$ 317,615,000
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
Options Clearing Corporation [Domain]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Customer securities under customer margin loans agreement available to be repledged	$ 484,900,000