Collateralized Transactions - Additional Information (Narrative) (Details)	3 Months Ended
	Mar. 31, 2019 USD ($) dealer	Dec. 31, 2018 USD ($)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gross Amounts of Recognized Liabilities	$ 369,109,000	$ 566,357,000
Gross Amounts of Recognized Liabilities	232,370,000	146,815,000
Bank call loans	0	15,000,000
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	186,000,000
Outstanding letters of credit	0
Amounts pledged	390,832,000	517,951,000
Carrying value of securities owned by the Company loaned or pledged	$ 0	20,200,000
Number of broker-dealers | dealer	6
Receivable from brokers and clearing organizations	$ 93,100,000
Securities Sold under Agreements to Repurchase, Asset	100,488,000	82,139,000
Securities Sold under Agreements to Repurchase, Amount Not Offset Against Collateral	268,621,000	484,218,000
Securities Sold under Agreements to Repurchase, Collateral, Right to Reclaim Securities	268,156,000	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	465,000	3,896,000
Securities Loaned, Asset	0	0
Securities Loaned, Amount Not Offset Against Collateral	232,370,000	146,815,000
Securities Loaned, Collateral, Right to Reclaim Securities	225,992,000	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	6,378,000	7,583,000
Derivative Liability, Securities Sold under Agreements to Repurchase, Securities Loaned, Gross	601,479,000	713,172,000
Derivative Liability, Securities Sold under Agreements to Repurchase, Securities Loaned, Asset	100,488,000	82,139,000
Derivative Liability, Securities Sold under Agreements to Resell, Securities Loaned, Amount Not Offset Against Collateral	500,991,000	631,033,000
Derivative Liability, Securities Sold under Agreements to Repurchase, Securities Loaned, Collateral, Right to Reclaim Securities	494,148,000	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	6,843,000	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	100,900,000	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	36,600,000	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	100,900,000	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	107,300,000	$ 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	601,479,000
Short-term Debt [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Collateralized loans, collateralized by firm	18,600,000
Collateralized loans, collateralized by customer securities	$ 1,600,000
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	$ 219,400,000