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Acquisitions (Narrative) (Details) - USD ($) |  |  | 1 Months Ended | 9 Months Ended |  |
--- | --- | --- | --- | --- | --- | ---
Dec. 01, 2019 | Sep. 05, 2019 | Jan. 31, 2019 | Sep. 30, 2020 | Sep. 30, 2019 | Jan. 02, 2020 | Dec. 31, 2019
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Purchase of StockCross common stock |  |  |  |  | $ 3,425,000 |  |  |
Treasury stock sales - StockCross |  |  |  |  | $ 172,000 |  |  |
Estimated effective tax rate |  |  |  |  | 28.00% |  |  |
Goodwill |  |  |  | 1,989,000 |  |  | $ 1,989,000 | [1]
Return of capital distribution |  |  |  |  | $ 1,600,000 |  |  |
Unrelated party [Member] |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Purchase of StockCross common stock |  |  |  |  | $ 3,665,000 |  |  |
StockCross [Member] |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Ownership percentage acquired |  | 15.00% | 15.00% |  | 15.00% |  |  |
Purchase price |  |  | $ 29,750,000 |  |  |  |  |
Purchase price in shares as restricted stock |  |  | 3,298,774 |  |  |  |  |
Estimated effective tax rate |  |  |  |  | 28.00% |  |  |
Goodwill |  |  |  |  |  |  |  |
Return of capital distribution |  | $ 1,600,000 |  |  |  |  |  |
Proceeds from return of captial |  | $ 241,000 |  |  |  |  |  |
When StockCross Merged with MSCO [Member] |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Ownership percentage acquired |  |  |  |  |  | 85.00% |  |
WP [Member] |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Ownership percentage acquired | 100.00% |  |  |  |  |  |  |
Estimated effective tax rate | 28.00% |  |  |  |  |  |  |
Goodwill | $ 1,989,000 |  |  |  |  |  |  |
[1] | Statement of financial condition as of December 31, 2019 represents the pro forma combination of Siebert and StockCross balances. See "Note 3 - Acquisitions" for additional detail.

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