Collateralized Transactions - Additional Information (Narrative) (Details)	9 Months Ended
	Sep. 30, 2018 USD ($) dealer	Dec. 31, 2017 USD ($)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gross Amounts of Recognized Liabilities	$ 705,491,000	$ 786,532,000
Gross Amounts of Recognized Liabilities	197,332,000	180,270,000
Bank call loans	3,000,000	118,300,000
Customer Securities for which Entity has Right to Sell or Repledge, Fair Value	1,100,000,000
Customer securities under customer margin loans agreement available to be repledged	169,900,000
Outstanding letters of credit	0
Amounts pledged	738,835,000	655,683,000
Carrying value of securities owned by the Company loaned or pledged	$ 3,300,000	97,200,000
Number of broker-dealers | dealer	5
Receivable from brokers and clearing organizations	$ 84,300,000
Securities Sold under Agreements to Repurchase, Asset	99,481,000	200,054,000
Securities Sold under Agreements to Repurchase, Amount Not Offset Against Collateral	606,010,000	586,478,000
Securities Sold under Agreements to Repurchase, Collateral, Right to Reclaim Securities	603,961,000	585,289,000
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	2,049,000	1,189,000
Securities Loaned, Asset	0	0
Securities Loaned, Amount Not Offset Against Collateral	197,332,000	180,270,000
Securities Loaned, Collateral, Right to Reclaim Securities	189,961,000	170,176,000
Securities Loaned, Collateral, Right to Reclaim Cash	0	0
Securities Loaned, Offset Against Collateral, Net of Not Subject to Master Netting Arrangement, Policy Election	7,371,000	10,094,000
Derivative Liability, Securities Sold under Agreements to Repurchase, Securities Loaned, Gross	902,823,000	966,802,000
Derivative Liability, Securities Sold under Agreements to Repurchase, Securities Loaned, Asset	99,481,000	200,054,000
Derivative Liability, Securities Sold under Agreements to Resell, Securities Loaned, Amount Not Offset Against Collateral	803,342,000	766,748,000
Derivative Liability, Securities Sold under Agreements to Repurchase, Securities Loaned, Collateral, Right to Reclaim Securities	793,922,000	755,465,000
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	9,420,000	11,283,000
Reverse Repurchase Agreements
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities received as collateral under securities borrowed transaction with market value	100,600,000	221,600,000
Securities Loaned [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Customer securities under customer margin loans agreement available to be repledged	18,100,000	30,900,000
Repurchase Agreements [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Customer securities under customer margin loans agreement available to be repledged	100,600,000	221,600,000
Securities Borrowed Transactions
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities received as collateral under securities borrowed transaction with market value	110,700,000	$ 127,200,000
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	902,823,000
Short-term Debt [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Collateralized loans, collateralized by firm	3,300,000
Collateralized loans, collateralized by customer securities	$ 311,800,000
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	$ 287,000,000