Collateralized Transactions - Narrative (Details) $ in Thousands	9 Months Ended
Sep. 30, 2017 USD ($) broker-dealer	Dec. 31, 2016 USD ($)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Bank call loans	$ 130,100	$ 145,800
Customer securities under customer margin loans available to be pledged	1,000,000
Customer securities under customer margin loans agreement available to be repledged	145,100
Customer securities deposited to secure obligations and margin requirements under option contracts	264,500
Amounts pledged	642,501	438,385
Carrying value of securities owned by the Company loaned or pledged	$ 143,000	138,600
Receivable from brokers and clearing organizations, number of major broker-dealers \| broker-dealer	5
Receivable from brokers and clearing organizations	$ 122,500
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	2 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	$ 154,500	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	241,000	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	27,900	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	241,000	$ 24,000
Corporate [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Collateralized loans, collateralized by firm	143,000
Other Than Corporate Customer [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Collateralized loans, collateralized by customer securities	$ 188,400

Credit concentration receivable from major broker dealers.

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- Definition

Margin deposited with option clearing corporation.

+ References

No definition available.

+ Details

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- Definition

Fair value of loans collateralized by customer securities.

+ References

No definition available.

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- Definition

Fair value of loans collateralized by firm.

+ References

No definition available.

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- Definition

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

+ References

No definition available.

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- Definition

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

+ References

No definition available.

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Settlement with Financial Institution, Period to Complete Transactions

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- Definition

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

+ References

No definition available.

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- Definition

Fair value as of the balance sheet date of customer securities accepted as collateral by the entity that it is permitted by contract or custom to sell or re-pledge to counterparties as collateral.

+ References

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- Definition

Fair value as of the balance sheet date of customer securities accepted as collateral by the entity that it has sold or re-pledged to counterparties as collateral.

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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- Definition

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
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5086-111524

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- Definition

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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 50
-Paragraph 1A
-Subparagraph (b)(1)(i)
-URI http://asc.fasb.org/extlink&oid=64858031&loc=SL6224234-111729

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=66007379&loc=d3e113888-111728

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