Collateralized Transactions - Narrative (Details) $ in Thousands	6 Months Ended
Jun. 30, 2017 USD ($) broker-dealer	Dec. 31, 2016 USD ($)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities purchased under agreements to resell, fair value	$ 3,617	$ 24,006
Fair Value Of Securities Repurchase Agreement	0
U.S. Treasury and agency securities		378,084
Bank call loans	230,400	145,800
Customer securities under customer margin loans available to be pledged	1,200,000
Customer securities under customer margin loans agreement available to be repledged	114,800
Customer securities deposited to secure obligations and margin requirements under option contracts	235,800
Amounts pledged	644,359	438,385
Carrying value of securities owned by the Company loaned or pledged	$ 168,100	138,600
Receivable from brokers and clearing organizations, number of major broker-dealers \| broker-dealer	3
Receivable from brokers and clearing organizations	$ 79,400
Securities Loaned, Gross		179,875
Minimum [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Settlement with Financial Institution, Period to Complete Transactions	1 day
Maximum [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Settlement with Financial Institution, Period to Complete Transactions	3 days
Securities Borrowed [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities received as collateral under securities borrowed transaction with market value	$ 153,300	148,700
Reverse Repurchase Agreement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities received as collateral under securities borrowed transaction with market value	133,200	24,000
Securities Loaned [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Customer securities under customer margin loans agreement available to be repledged	20,900	37,400
Repurchase Agreements [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Customer securities under customer margin loans agreement available to be repledged	133,200	$ 24,000
Corporate [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Collateralized loans, collateralized by firm	168,100
Other Than Corporate Customer [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Collateralized loans, collateralized by customer securities	242,300
Maturity Overnight and Open [Member] \| US Treasury and Government [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
U.S. Treasury and agency securities	$ 586,507

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Margin deposited with option clearing corporation.

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Fair value of loans collateralized by customer securities.

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Fair value of loans collateralized by firm.

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Fair value of securities repurchase agreement.

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Receivables from Brokers-Dealers and Clearing Organizations, Number of Major Broker-Dealers

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

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Amounts due on short-term bank loans secured by customers' securities on the balance sheet date.

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The carrying amount (fair value) as of the balance sheet date of securities held that had been pledged by counterparties under financing and lending arrangements, pursuant to which the entity has the right by agreement or custom to sell or re-pledge such securities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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Amount, before effects of master netting arrangements, of securities loaned to entities in exchange for collateral. Includes liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement.

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Fair value of financial instrument held by the entity for its own account (proprietary securities) for trading or investment purposes that are carried at fair value and pledged to counterparties as collateral for financing transactions.

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