Collateralized Transactions - Additional Information (Narrative) (Details)	9 Months Ended
	Sep. 30, 2019 USD ($) dealer	Dec. 31, 2018 USD ($)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gross Amounts of Recognized Liabilities	$ 345,193,000	$ 566,357,000
Gross Amounts of Recognized Liabilities	209,298,000	146,815,000
Bank call loans	0	15,000,000
Customer Securities for which Entity has Right to Sell or Repledge, Fair Value	995,000,000
Customer securities under customer margin loans agreement available to be repledged	185,000,000
Outstanding letters of credit	0
Amounts pledged	359,151,000	517,951,000
Carrying value of securities owned by the Company loaned or pledged	$ 0	20,200,000
Number of broker-dealers | dealer	4
Receivable from brokers and clearing organizations	$ 72,800,000
Securities Sold under Agreements to Repurchase, Asset	91,473,000	82,139,000
Securities Sold under Agreements to Repurchase, Amount Not Offset Against Collateral	253,720,000	484,218,000
Securities Sold under Agreements to Repurchase, Collateral, Right to Reclaim Securities	252,401,000	480,322,000
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	1,319,000	3,896,000
Securities Loaned, Asset	0	0
Securities Loaned, Amount Not Offset Against Collateral	209,298,000	146,815,000
Securities Loaned, Collateral, Right to Reclaim Securities	203,747,000	139,232,000
Securities Loaned, Collateral, Right to Reclaim Cash	0	0
Securities Loaned, Offset Against Collateral, Net of Not Subject to Master Netting Arrangement, Policy Election	5,551,000	7,583,000
Derivative Liability, Securities Sold under Agreements to Repurchase, Securities Loaned, Gross	554,491,000	713,172,000
Derivative Liability, Securities Sold under Agreements to Repurchase, Securities Loaned, Asset	91,473,000	82,139,000
Derivative Liability, Securities Sold under Agreements to Resell, Securities Loaned, Amount Not Offset Against Collateral	463,018,000	631,033,000
Derivative Liability, Securities Sold under Agreements to Repurchase, Securities Loaned, Collateral, Right to Reclaim Securities	456,148,000	619,554,000
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	6,870,000	11,479,000
Reverse Repurchase Agreements
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities received as collateral under securities borrowed transaction with market value	91,500,000	83,000,000
Securities Loaned [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Customer securities under customer margin loans agreement available to be repledged	17,300,000	27,600,000
Repurchase Agreements [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Customer securities under customer margin loans agreement available to be repledged	91,500,000	83,000,000
Securities Borrowed Transactions
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities received as collateral under securities borrowed transaction with market value	100,800,000	$ 104,900,000
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	$ 554,491,000
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	$ 259,900,000