Fair Value Measurements (Schedule of Fair Value Hierarchy for Assets and Liabilities Measured at Fair Value) (Details) - USD ($)		Jun. 30, 2021	Dec. 31, 2020
Assets
Securities owned, at fair value		$ 4,275,000	$ 2,623,000
Liabilities
Securities sold, not yet purchased, at fair value		49,000	21,000
Equity Securities [Member]
Assets
Securities owned, at fair value		2,154,000	479,000
Liabilities
Securities sold, not yet purchased, at fair value		27,000	21,000
Level 1 [Member]
Assets
Securities owned, at fair value		3,113,000	2,374,000
Liabilities
Securities sold, not yet purchased, at fair value
Level 1 [Member] | Equity Securities [Member]
Assets
Securities owned, at fair value		1,098,000	345,000
Liabilities
Securities sold, not yet purchased, at fair value
Level 2 [Member]
Assets
Securities owned, at fair value		1,162,000	249,000
Liabilities
Securities sold, not yet purchased, at fair value		49,000	21,000
Level 2 [Member] | Equity Securities [Member]
Assets
Securities owned, at fair value		1,056,000	134,000
Liabilities
Securities sold, not yet purchased, at fair value		27,000	21,000
Level 3 [Member]
Assets
Securities owned, at fair value
Liabilities
Securities sold, not yet purchased, at fair value
Level 3 [Member] | Equity Securities [Member]
Assets
Securities owned, at fair value
Liabilities
Securities sold, not yet purchased, at fair value
U.S. government securities [Member]
Assets
Securities owned, at fair value	[1]	2,015,000	2,029,000
U.S. government securities [Member] | Level 1 [Member]
Assets
Securities owned, at fair value	[1]	2,015,000	2,029,000
U.S. government securities [Member] | Level 2 [Member]
Assets
Securities owned, at fair value	[1]
U.S. government securities [Member] | Level 3 [Member]
Assets
Securities owned, at fair value	[1]
Certificates of Deposit [Member]
Assets
Securities owned, at fair value		91,000	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	91,000
Certificates of Deposit [Member] | Level 3 [Member]
Assets
Securities owned, at fair value
Corporate bonds [Member]
Assets
Securities owned, at fair value		15,000	24,000
Corporate bonds [Member] | Level 1 [Member]
Assets
Securities owned, at fair value
Corporate bonds [Member] | Level 2 [Member]
Assets
Securities owned, at fair value		15,000	24,000
Corporate bonds [Member] | Level 3 [Member]
Assets
Securities owned, at fair value
Unit investment trust [Member]
Liabilities
Securities sold, not yet purchased, at fair value		22,000
Unit investment trust [Member] | Level 1 [Member]
Liabilities
Securities sold, not yet purchased, at fair value
Unit investment trust [Member] | Level 2 [Member]
Liabilities
Securities sold, not yet purchased, at fair value		22,000
Unit investment trust [Member] | Level 3 [Member]
Liabilities
Securities sold, not yet purchased, at fair value

[1] As of June 30, 2021 and December 31, 2020, these U.S. government securities mature on 08/31/21