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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