Collateralized Transactions - Additional Information (Narrative) (Details)	6 Months Ended
Jun. 30, 2018 USD ($) dealer	Dec. 31, 2017 USD ($)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gross Amounts of Recognized Liabilities		$ 786,532,000
Gross Amounts of Recognized Liabilities		180,270,000
Bank call loans	$ 107,500,000	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	161,200,000
Outstanding letters of credit	0
Amounts pledged	692,286,000	655,683,000
Carrying value of securities owned by the Company loaned or pledged	$ 92,800,000	97,200,000
Number of broker-dealers \| dealer	4
Receivable from brokers and clearing organizations	$ 77,200,000
Reverse Repurchase Agreements
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Customer securities under customer margin loans agreement available to be repledged	95,100,000	221,600,000
Securities received as collateral under securities borrowed transaction with market value	95,100,000	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	26,600,000	30,900,000
Securities received as collateral under securities borrowed transaction with market value	118,200,000	$ 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	893,760,000
Short-term Debt [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Collateralized loans, collateralized by firm	93,200,000
Collateralized loans, collateralized by customer securities	442,600,000
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	694,162,000
Equity Securities [Member] \| Maturity Overnight and on Demand [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gross Amounts of Recognized Liabilities	$ 199,598,000
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	$ 364,700,000

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Securities owned and pledged as collateral amount not eligible to be repledged by counterparty carrying value.

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