Fair Value Measurements (Schedule of Fair Value Hierarchy for Assets and Liabilities Measured at Fair Value) (Details) - USD ($)	Dec. 31, 2021	Dec. 31, 2020
Assets
Cash and securities segregated for regulatory purposes	$ 326,826,000	$ 324,924,000
Securities owned, at fair value	3,991,000	2,623,000
Liabilities
Securities sold, not yet purchased, at fair value	24,000	21,000
Equity Securities [Member]
Assets
Securities owned, at fair value	922,000	479,000
Liabilities
Securities sold, not yet purchased, at fair value	24,000	21,000
Level 1 [Member]
Assets
Securities owned, at fair value	3,455,000	2,374,000
Liabilities
Securities sold, not yet purchased, at fair value
Level 1 [Member] | Equity Securities [Member]
Assets
Securities owned, at fair value	489,000	345,000
Liabilities
Securities sold, not yet purchased, at fair value
Level 2 [Member]
Assets
Securities owned, at fair value	536,000	249,000
Liabilities
Securities sold, not yet purchased, at fair value	24,000	21,000
Level 2 [Member] | Equity Securities [Member]
Assets
Securities owned, at fair value	433,000	134,000
Liabilities
Securities sold, not yet purchased, at fair value	24,000	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	2,966,000	2,029,000	[1]
U.S. government securities [Member] | Level 1 [Member]
Assets
Securities owned, at fair value	2,966,000	2,029,000	[1]
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	12,000	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	12,000	24,000
Corporate bonds [Member] | Level 3 [Member]
Assets
Securities owned, at fair value

[1] As of December 31, 2021 and 2020, the U.S. government securities had maturity dates of August 15, 2024 and August 31, 2021, respectively.