Financial Instruments with Off-Balance Sheet Risk (Narrative) (Details) $ in Millions	Sep. 30, 2021 USD ($)
Financial Instruments With Off-balance-sheet Risk And Concentrations Of Credit Risk
Margin loans extended	$ 1,200.0
Receivables from customers	$ 85.4

Amount due from customers for fees and charges arising from transactions related to the entity's brokerage activities and operations.

No definition available.

Name:	SIEB_ReceivablesFromCustomers1
Namespace Prefix:	SIEB_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant