Collateralized Transactions - Additional Information (Narrative) (Details)	6 Months Ended
	Jun. 30, 2020 USD ($) dealer	Dec. 31, 2019 USD ($)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gross Amounts of Recognized Liabilities	$ 382,005,000	$ 343,192,000
Gross Amounts of Recognized Liabilities	204,339,000	234,343,000
Bank call loans	13,000,000	0
Customer Securities for which Entity has Right to Sell or Repledge, Fair Value	1,200,000,000
Customer securities under customer margin loans agreement available to be repledged	178,100,000
Outstanding letters of credit	0
Amounts pledged	357,323,000	357,120,000
Carrying value of securities owned by the Company loaned or pledged	$ 17,000,000.0
Number of broker-dealers | dealer	3
Receivable from brokers and clearing organizations	$ 76,900,000
Securities Sold under Agreements to Repurchase, Asset	227,850,000	55,927,000
Securities Sold under Agreements to Repurchase, Amount Not Offset Against Collateral	154,155,000	287,265,000
Securities Sold under Agreements to Repurchase, Collateral, Right to Reclaim Securities	153,701,000	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	454,000	2,001,000
Securities Loaned, Asset	0	0
Securities Loaned, Amount Not Offset Against Collateral	204,339,000	234,343,000
Securities Loaned, Collateral, Right to Reclaim Securities	202,253,000	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	2,086,000	5,795,000
Derivative Liability, Securities Sold under Agreements to Repurchase, Securities Loaned, Gross	586,344,000	577,535,000
Derivative Liability, Securities Sold under Agreements to Repurchase, Securities Loaned, Asset	227,850,000	55,927,000
Derivative Liability, Securities Sold under Agreements to Resell, Securities Loaned, Amount Not Offset Against Collateral	358,494,000	521,608,000
Derivative Liability, Securities Sold under Agreements to Repurchase, Securities Loaned, Collateral, Right to Reclaim Securities	355,954,000	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	2,540,000	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	227,900,000	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	22,300,000	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	227,900,000	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	123,200,000	$ 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	$ 586,344,000
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	$ 364,000,000.0