Collateralized Transactions - Additional Information (Narrative) (Details)		12 Months Ended
		Dec. 31, 2017 USD ($) dealer	Dec. 31, 2016 USD ($)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Bank call loans		$ 118,300,000	$ 145,800,000
Customer securities under customer margin loans available to be pledged		1,200,000,000
Customer securities deposited to secure obligations and margin requirements under option contracts		237,000,000
Outstanding letters of credit		0
Gross Amounts of Recognized Liabilities			378,084,000
Gross Amounts of Recognized Liabilities	[1]		179,875,000
Amounts pledged		655,683,000	438,385,000
Carrying value of securities owned by the Company loaned or pledged		$ 97,200,000	138,600,000
Number of broker-dealers | dealer		4
Receivable from brokers and clearing organizations		$ 94,000,000
Reverse Repurchase Agreements
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities received as collateral under securities borrowed transaction with market value		221,600,000	24,000,000
Securities Borrowed Transactions
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Customer securities under customer margin loans agreement available to be repledged		30,900,000	37,400,000
Securities received as collateral under securities borrowed transaction with market value		127,200,000	148,700,000
Repurchase Agreements [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Customer securities under customer margin loans agreement available to be repledged		221,600,000	$ 24,000,000
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		966,802,000
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		786,532,000
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]	$ 180,270,000
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		$ 97,800,000
Customer securities under customer margin loans agreement available to be repledged		198,300,000
Securities Loaned [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Customer securities under customer margin loans agreement available to be repledged		$ 139,200,000

[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.