Collateralized Transactions - Additional Information (Narrative) (Details)		12 Months Ended
		Dec. 31, 2018 USD ($) dealer	Dec. 31, 2017 USD ($)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Bank call loans		$ 15,000,000	$ 118,300,000
Customer securities under customer margin loans available to be pledged		1,000,000,000
Customer securities deposited to secure obligations and margin requirements under option contracts		460,200,000
Outstanding letters of credit		0
Gross Amounts of Recognized Liabilities			786,532,000
Gross Amounts of Recognized Liabilities	[1]		180,270,000
Amounts pledged		518,000,000	655,700,000
Carrying value of securities owned by the Company loaned or pledged		$ 20,200,000	97,200,000
Number of broker-dealers | dealer		5
Receivable from brokers and clearing organizations		$ 86,200,000
Reverse Repurchase Agreements
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities received as collateral under securities borrowed transaction with market value		83,000,000	221,600,000
Securities Borrowed Transactions
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Customer securities under customer margin loans agreement available to be repledged		27,600,000	30,900,000
Securities received as collateral under securities borrowed transaction with market value		104,900,000	127,200,000
Repurchase Agreements [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Customer securities under customer margin loans agreement available to be repledged		83,000,000	$ 221,600,000
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		713,172,000
U.S. treasury, agency and sovereign obligations | Maturity Overnight and on Demand [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gross Amounts of Recognized Liabilities		566,357,000
Equity Securities [Member] | Maturity Overnight and on Demand [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gross Amounts of Recognized Liabilities	[2]	$ 146,815,000
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
Commercial Banks
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Collateralized loans, collateralized by firm		$ 18,600,000
Customer securities under customer margin loans agreement available to be repledged		1,600,000
Securities Loaned [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Customer securities under customer margin loans agreement available to be repledged		$ 112,600,000

[1] Included in payable to brokers, dealers and clearing organizations on the consolidated balance sheet.

[2] Included in payable to brokers, dealers and clearing organizations on the consolidated balance sheet.