Revenue Recognition (Disaggregation of revenue) (Details) - USD ($)	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Disaggregation of Revenue [Line Items]
Total revenue from Principal transactions	$ 2,634,000	$ 154,000	$ 7,838,000	$ 360,000
Total revenue from Commissions and fees	2,465,000	1,077,000	7,651,000	3,295,000
Total revenue from Margin interest, marketing and distribution fees	2,731,000	1,850,000	7,953,000	4,477,000
Total revenue from Interest	34,000	4,000	69,000	9,000
Total revenue from Advisory fees	20,000	4,000	38,000	16,000
Total Revenue	7,884,000	3,089,000	23,549,000	8,157,000
Non StockCross [Member]
Disaggregation of Revenue [Line Items]
Total revenue from Interest	34,000	4,000	69,000	9,000
Total revenue from Advisory fees	20,000	4,000	38,000	16,000
Principal transactions [Member] \| Non StockCross [Member]
Disaggregation of Revenue [Line Items]
Total revenue from Principal transactions	437,000	154,000	1,514,000	360,000
Principal transactions [Member] \| Assets acquired from StockCross [Member]
Disaggregation of Revenue [Line Items]
Total revenue from Principal transactions	2,197,000		6,324,000
Commissions and fees [Member] \| Non StockCross [Member]
Disaggregation of Revenue [Line Items]
Total revenue from Commissions and fees	2,068,000	1,077,000	6,185,000	3,295,000
Commissions and fees [Member] \| Assets acquired from StockCross [Member]
Disaggregation of Revenue [Line Items]
Total revenue from Commissions and fees	397,000		1,466,000
Margin interest, marketing and distribution fees [Member] \| Non StockCross [Member]
Disaggregation of Revenue [Line Items]
Total revenue from Margin interest, marketing and distribution fees	2,409,000	1,850,000	7,049,000	4,477,000
Margin interest, marketing and distribution fees [Member] \| Assets acquired from StockCross [Member]
Disaggregation of Revenue [Line Items]
Total revenue from Margin interest, marketing and distribution fees	$ 322,000		$ 904,000

Advisory fees revenue.

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- Definition

Amount of fees and financial services revenue classified as other.

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No definition available.

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- Definition

Revenue from commissions and fees.

+ References

No definition available.

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- Definition

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+ References

No definition available.

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Amount, after discount, accretion and premium amortization, of interest income and dividend income classified as other.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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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.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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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).

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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