Fair Value Measurements (Schedule of Fair Value Hierarchy for Assets and Liabilities Measured at Fair Value) (Details) - USD ($)	Sep. 30, 2020		Dec. 31, 2019
Assets
Securities owned, at fair value	$ 3,480,000		$ 3,018,000
Liabilities
Securities sold, not yet purchased, at fair value	25,000		116,000	[1]
Equity Securities [Member]
Assets
Securities owned, at fair value	1,108,000		986,000
Liabilities
Securities sold, not yet purchased, at fair value	25,000		116,000
Level 1 [Member]
Assets
Securities owned, at fair value	2,926,000		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	765,000		453,000
Liabilities
Securities sold, not yet purchased, at fair value			88,000
Level 2 [Member]
Assets
Securities owned, at fair value	554,000		270,000
Liabilities
Securities sold, not yet purchased, at fair value	25,000		28,000
Level 2 [Member] | Equity Securities [Member]
Assets
Securities owned, at fair value	343,000		245,000
Liabilities
Securities sold, not yet purchased, at fair value	25,000		28,000
Level 3 [Member]
Assets
Securities owned, at fair value			288,000
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
Certificates of Deposit [Member]
Assets
Cash and cash equivalents	142,000		142,000
Securities owned, at fair value	91,000
Certificates of Deposit [Member] | Level 1 [Member]
Assets
Cash and cash equivalents
Securities owned, at fair value
Certificates of Deposit [Member] | Level 2 [Member]
Assets
Cash and cash equivalents	142,000		142,000
Securities owned, at fair value	91,000
Certificates of Deposit [Member] | Level 3 [Member]
Assets
Cash and cash equivalents
Securities owned, at fair value
U.S. government securities [Member]
Assets
Segregated securities			1,311,000
Securities owned, at fair value	2,028,000	[2]	2,007,000
U.S. government securities [Member] | Level 1 [Member]
Assets
Segregated securities			1,311,000
Securities owned, at fair value	2,028,000	[2]	2,007,000
U.S. government securities [Member] | Level 2 [Member]
Assets
Segregated securities
Securities owned, at fair value		[2]
U.S. government securities [Member] | Level 3 [Member]
Assets
Segregated securities
Securities owned, at fair value		[2]
Corporate bonds [Member]
Assets
Securities owned, at fair value	120,000		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	120,000		25,000
Corporate bonds [Member] | Level 3 [Member]
Assets
Securities owned, at fair value
Mutual Fund [Member]
Assets
Securities owned, at fair value	133,000
Mutual Fund [Member] | Level 1 [Member]
Assets
Securities owned, at fair value	133,000
Mutual Fund [Member] | Level 2 [Member]
Assets
Securities owned, at fair value
Mutual Fund [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 September 30, 2020, U.S. government securities mature on 08/31/2021