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Related Party Transactions (Narrative) (Details) - USD ($) $ / shares in Units, $ in Thousands |  |  |  | 1 Months Ended | 3 Months Ended | 9 Months Ended | 12 Months Ended |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Aug. 29, 2018 | May 25, 2018 | Sep. 29, 2017 | Jul. 31, 2018 | Jul. 31, 2017 | Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017 | Dec. 31, 2015 | Dec. 31, 2013 | Dec. 31, 2017 | Jan. 31, 2017 | Dec. 31, 2016 | Oct. 03, 2016
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Securities sold under agreements to repurchase |  |  |  |  |  | $ 4,506,628 |  | $ 4,506,628 |  |  |  | $ 1,692,279 |  |  |
Fair value of securities pledged as collateral under repurchase agreements |  |  |  |  |  | 4,528,074 |  | 4,528,074 |  |  |  | 1,708,154 |  |  |
Redeemable financial instrument |  |  |  |  |  | 16,732 |  | 16,732 |  |  |  | 16,732 |  |  |
Cash and cash equivalents |  |  |  |  |  | 5,166 | $ 22,133 | 5,166 | $ 22,133 |  |  | 22,933 |  | $ 15,216 |
Related Party Transaction, Expenses from Transactions with Related Party |  |  |  |  |  | 24 |  | $ 72 |  |  |  |  |  |  |
Company matching percent |  |  |  |  |  |  |  | 50.00% |  |  |  |  |  |  |
Percent of employees gross salary qualifying for matching contributions |  |  |  |  |  |  |  | 3.00% |  |  |  |  |  |  |
Company plan contributions |  |  |  |  |  | 46 | 41 | $ 164 | $ 177 |  |  |  |  |  |
Purchase of common stock, shares | 17,555 | 2,774 |  |  |  |  |  |  | 5,720 |  |  |  |  |  |
Treasury Stock, Value, Acquired, Par Value Method | $ 176 | $ 33 |  |  |  |  |  |  |  |  |  |  |  |  |
Shares repurchased, price per share | $ 10 | $ 12 |  |  |  |  |  |  |  |  |  |  |  |  |
EBC Family Trust [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Notes payable, related parties |  |  |  |  |  |  |  |  |  |  | $ 2,400 |  |  |  |
Value of shares issued |  |  |  |  |  |  |  |  |  |  | 1,600 |  |  |  |
Related Party Transaction, Amounts of Transaction |  |  |  |  |  |  |  |  |  |  | $ 4,000 |  |  |  |
TBBK [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Securities sold under agreements to repurchase |  |  |  |  |  | 64,534 |  | 64,534 |  |  |  | 64,370 |  |  |
Fair value of securities pledged as collateral under repurchase agreements |  |  |  |  |  | 66,455 |  | 66,455 |  |  |  | 66,862 |  |  |
Interest Expense, Related Party |  |  |  |  |  | 480 | 370 | 1,369 | $ 930 |  |  |  |  |  |
Cash and cash equivalents |  |  |  |  |  | 0 |  | 0 |  |  |  | 81 |  |  |
Daniel G. Cohen [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transaction, Expenses from Transactions with Related Party |  |  |  |  |  |  |  | 600 |  |  |  |  |  |  |
JKD Investor [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Redeemable financial instrument |  |  |  |  |  | 6,732 |  | 6,732 |  |  |  | 6,732 | $ 1,000 |  | $ 6,000
CBF Cohen Bros. Financial [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transaction, Amounts of Transaction |  |  | $ 8,000 |  |  |  |  |  |  |  |  |  |  |  |
DGC Family Fintech Trust/CBF [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Redeemable financial instrument |  |  | 10,000 |  |  | 10,000 |  | 10,000 |  |  |  | $ 10,000 |  |  |
Related Party Transaction, Amounts of Transaction |  |  | 10,000 |  |  |  |  |  |  |  |  |  |  |  |
DGC Family Fintech Trust [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transaction, Amounts of Transaction |  |  | $ 2,000 |  |  |  |  |  |  |  |  |  |  |  |
Fin Tech Acquisition Corp II [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related parties, agreement period for accounting and support services |  |  |  |  | 24 months |  |  |  |  |  |  |  |  |  |
Principal transactions and other income |  |  |  |  |  | 12 | $ 1 | 17 | $ 1 |  |  |  |  |  |
Board of Directors, Vice Chairman [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Reduction in rent and utility expenses |  |  |  |  |  | 11 |  | $ 0 |  |  |  |  |  |  |
Lease Expiration Date |  |  |  |  |  |  |  | May 31, 2017 |  |  |  |  |  |  |
COHN, LLC [Member] | Fin Tech Acquisition Corp II [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Principal transactions and other income |  |  |  |  |  | $ 397 |  | $ 397 |  |  |  |  |  |  |
COHN, LLC [Member] | Fin Tech Acquisition Corp II [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquired publicly traded shares |  |  |  | $ 2,513 |  |  |  |  |  |  |  |  |  |  |
Contingent Convertible Senior Notes [Member] | Edward E. Cohen IRA [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Convertible notes purchased |  |  |  |  |  |  |  |  |  | $ 4,386 |  |  |  |  |
10b5-1 Plan [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Purchase of common stock, shares |  |  |  |  |  | 10,390 | 5,440 | 40,202 | 5,720 |  |  |  |  |  |
Treasury Stock, Value, Acquired, Par Value Method |  |  |  |  |  | $ 105 | $ 63 | $ 425 | $ 66 |  |  |  |  |  |

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