Revenue Recognition (Disaggregation of revenue) (Details) - USD ($)	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
Disaggregation of Revenue [Line Items]
Total revenue from Principal transactions	$ 1,610,000	$ 2,941,000
Total revenue from Commissions and fees	1,864,000	2,674,000
Total revenue from Margin interest, marketing and distribution fees	2,772,000	2,535,000
Total revenue from Advisory fees	168,000	16,000
Total revenue from Interest	15,000	11,000
Total revenue	6,429,000	8,177,000
Legacy Siebert [Member]
Disaggregation of Revenue [Line Items]
Total revenue from Advisory fees	168,000	16,000
Total revenue from Interest	15,000	11,000
Principal transactions [Member] \| Legacy Siebert [Member]
Disaggregation of Revenue [Line Items]
Total revenue from Principal transactions	379,000	572,000
Principal transactions [Member] \| StockCross accounts [Member]
Disaggregation of Revenue [Line Items]
Total revenue from Principal transactions	1,231,000	2,369,000
Commissions and fees [Member] \| Legacy Siebert [Member]
Disaggregation of Revenue [Line Items]
Total revenue from Commissions and fees	1,580,000	1,955,000
Commissions and fees [Member] \| StockCross accounts [Member]
Disaggregation of Revenue [Line Items]
Total revenue from Commissions and fees	284,000	719,000
Margin interest, marketing and distribution fees [Member] \| Legacy Siebert [Member]
Disaggregation of Revenue [Line Items]
Total revenue from Margin interest, marketing and distribution fees	2,445,000	2,328,000
Margin interest, marketing and distribution fees [Member] \| StockCross accounts [Member]
Disaggregation of Revenue [Line Items]
Total revenue from Margin interest, marketing and distribution fees	$ 327,000	$ 207,000

Advisory fees revenue.

No definition available.

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- Definition

Revenue from commissions and fees.

+ References

No definition available.

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- Definition

Revenue from margin interest, marketing and distribution fees.

+ References

No definition available.

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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

Represents the total of interest and dividend income, including any amortization and accretion (as applicable) of discounts and premiums, earned from (1) loans and leases whether held-for-sale or held-in-portfolio; (2) investment securities; (3) federal funds sold; (4) securities purchased under agreements to resell; (5) investments in banker's acceptances, commercial paper, or certificates of deposit; (6) dividend income; or (7) other investments not otherwise specified herein.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04.1-5)
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- Definition

Amount of realized and unrealized gain (loss) resulting from the difference between acquisition price and selling price or fair value of trading assets and trading liabilities, and from the firm's direct investment activity, conducted separately from customer trading activities, including, but not limited to, investments in private equity, alternative investment products, real estate, and exchanges and memberships.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 940
-SubTopic 320
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=35711157&loc=d3e42567-110969

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- Definition

Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 22
-Subparagraph (a)
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
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Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
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-URI http://asc.fasb.org/extlink&oid=115929826&loc=d3e8736-108599

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03(1))
-URI http://asc.fasb.org/extlink&oid=116634182&loc=SL114868664-224227

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