Fair Value Measurements (Schedule of Fair Value Hierarchy for Assets and Liabilities Measured at Fair Value) (Details) - USD ($)		Dec. 31, 2020	Dec. 31, 2019
Assets
Cash and securities segregated for regulatory purposes		$ 324,924,000	$ 224,924,000	[1]
Securities owned, at fair value		2,623,000	3,018,000
Liabilities
Securities sold, not yet purchased, at fair value		21,000	116,000	[1]
Equity Securities [Member]
Assets
Securities owned, at fair value		479,000	986,000
Liabilities
Securities sold, not yet purchased, at fair value		21,000	116,000
Level 1 [Member]
Assets
Securities owned, at fair value		2,374,000	2,460,000
Liabilities
Securities sold, not yet purchased, at fair value			88,000
Level 1 [Member] | Equity Securities [Member]
Assets
Securities owned, at fair value		345,000	453,000
Liabilities
Securities sold, not yet purchased, at fair value			88,000
Level 2 [Member]
Assets
Securities owned, at fair value		249,000	270,000
Liabilities
Securities sold, not yet purchased, at fair value		21,000	28,000
Level 2 [Member] | Equity Securities [Member]
Assets
Securities owned, at fair value		134,000	245,000
Liabilities
Securities sold, not yet purchased, at fair value		21,000	288,000
Level 3 [Member]
Assets
Securities owned, at fair value			280,000
Liabilities
Securities sold, not yet purchased, at fair value
Level 3 [Member] | Equity Securities [Member]
Assets
Securities owned, at fair value			288,000
Liabilities
Securities sold, not yet purchased, at fair value
U.S. government securities [Member]
Assets
Cash and securities segregated for regulatory purposes	[2]		1,311,000
Securities owned, at fair value	[3]	2,029,000	2,007,000
U.S. government securities [Member] | Level 1 [Member]
Assets
Cash and securities segregated for regulatory purposes	[2]		1,311,000
Securities owned, at fair value	[3]	2,029,000	2,007,000
U.S. government securities [Member] | Level 2 [Member]
Assets
Cash and securities segregated for regulatory purposes	[2]
Securities owned, at fair value	[3]
U.S. government securities [Member] | Level 3 [Member]
Assets
Cash and securities segregated for regulatory purposes	[2]
Securities owned, at fair value	[3]
Certificates of Deposit [Member]
Assets
Securities owned, at fair value		91,000
Certificates of Deposit [Member] | Level 1 [Member]
Assets
Securities owned, at fair value
Certificates of Deposit [Member] | Level 2 [Member]
Assets
Securities owned, at fair value		91,000
Certificates of Deposit [Member] | Level 3 [Member]
Assets
Securities owned, at fair value
Corporate bonds [Member]
Assets
Securities owned, at fair value		24,000	25,000
Corporate bonds [Member] | Level 1 [Member]
Assets
Securities owned, at fair value
Corporate bonds [Member] | Level 2 [Member]
Assets
Securities owned, at fair value		24,000	25,000
Corporate bonds [Member] | Level 3 [Member]
Assets
Securities owned, at fair value

[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.

[2] As of December 31, 2019, these U.S. government securities mature on 02/29/20

[3] As of December 31, 2020 and 2019, these U.S. government securities mature on 08/31/21