Collateralized Transactions - Additional Information (Narrative) (Details)		12 Months Ended
		Dec. 31, 2019 USD ($) dealer	Dec. 31, 2018 USD ($)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Bank call loans		$ 0	$ 15,000,000
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		234,400,000
Outstanding letters of credit		0
Gross Amounts of Recognized Liabilities			566,357,000
Gross Amounts of Recognized Liabilities	[1]		146,815,000
Amounts pledged		357,100,000	518,000,000
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		$ 61,300,000
Reverse Repurchase Agreements
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities received as collateral under securities borrowed transaction with market value		55,800,000	83,000,000
Securities Borrowed Transactions
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Customer securities under customer margin loans agreement available to be repledged		19,300,000	27,600,000
Securities received as collateral under securities borrowed transaction with market value		96,300,000	104,900,000
Repurchase Agreements [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Customer securities under customer margin loans agreement available to be repledged		55,800,000	$ 83,000,000
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		577,535,000
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		343,192,000
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]	$ 234,343,000
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
Securities Loaned [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Customer securities under customer margin loans agreement available to be repledged		$ 207,200,000

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

[2] As of December 31, 2018(Expressed in thousands) Gross Amounts Not Offseton the Balance Sheet GrossAmounts ofRecognizedAssets GrossAmountsOffset on the Balance Sheet Net Amountsof AssetsPresented onthe Balance Sheet FinancialInstruments CashCollateralReceived Net AmountReverse repurchase agreements$82,429 $(82,139) $290 $— $— $290Securities borrowed (1)108,144 — 108,144 (105,960) — 2,184Total$190,573 $(82,139) $108,434 $(105,960) $— $2,474