Acquisitions (Schedule of Statements of Operations (unaudited)) (Details) - USD ($)	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Revenue
Commissions and fees	$ 5,583,000	$ 2,268,000
Margin interest, marketing and distribution fees	3,294,000	3,556,000
Principal transactions	3,203,000	1,890,000
Interest income	1,331,000	1,173,000
Market making	470,000	563,000
Stock borrow / stock loan	444,000	581,000
Advisory fees	262,000	168,000
Other income	214,000	79,000
Total Revenue	14,801,000	10,278,000
Expenses
Employee compensation and benefits	7,291,000	4,528,000
Clearing fees, including execution costs	1,298,000	802,000
Technology and communications	981,000	422,000
Other general and administrative	854,000	733,000
Data processing	849,000	543,000
Rent and occupancy	727,000	531,000
Professional fees	655,000	883,000
Depreciation and amortization	448,000	194,000
Interest expense	76,000	21,000
Total Expenses	13,290,000	8,657,000
Income before provision for income taxes	1,511,000	1,621,000
Provision for income taxes	535,000	397,000
Net income / (loss)	$ 976,000	$ 1,224,000
Net income per share of common stock
Basic and diluted	$ 0.03	$ 0.04
Weighted average shares outstanding
Basic and diluted	30,459,804	30,459,804
Siebert [Member]
Revenue
Commissions and fees		$ 1,864,000
Margin interest, marketing and distribution fees		2,772,000
Principal transactions		1,610,000
Interest income		15,000
Market making
Stock borrow / stock loan
Advisory fees		168,000
Other income
Total Revenue		6,429,000
Expenses
Employee compensation and benefits		2,835,000
Clearing fees, including execution costs		654,000
Technology and communications		247,000
Other general and administrative		385,000
Data processing
Rent and occupancy		295,000
Professional fees		502,000
Depreciation and amortization		175,000
Interest expense
Total Expenses		5,093,000
Earnings of equity method investment in related party		39,000
Income before provision for income taxes		1,375,000
Provision for income taxes		369,000
Net income / (loss)		$ 1,006,000
Net income per share of common stock
Basic and diluted		$ 0.04
Weighted average shares outstanding
Basic and diluted		27,157,188
StockCross [Member]
Revenue
Commissions and fees		$ 404,000
Margin interest, marketing and distribution fees		784,000
Principal transactions		280,000
Interest income		1,158,000
Market making		563,000
Stock borrow / stock loan		581,000
Advisory fees
Other income		138,000
Total Revenue		3,908,000
Expenses
Employee compensation and benefits		1,693,000
Clearing fees, including execution costs		207,000
Technology and communications		175,000
Other general and administrative		348,000
Data processing		543,000
Rent and occupancy		236,000
Professional fees		381,000
Depreciation and amortization		19,000
Interest expense		21,000
Total Expenses		3,623,000
Earnings of equity method investment in related party
Income before provision for income taxes		285,000
Provision for income taxes		39,000
Net income / (loss)		$ 246,000
Net income per share of common stock
Basic and diluted		$ 0.04
Weighted average shares outstanding
Basic and diluted		6,152,500
Pro Forma Adjustments (unaudited) [Member]
Revenue
Commissions and fees
Margin interest, marketing and distribution fees
Principal transactions
Interest income
Market making
Stock borrow / stock loan
Advisory fees
Other income		(59,000)
Total Revenue		(59,000)
Expenses
Employee compensation and benefits
Clearing fees, including execution costs		(59,000)
Technology and communications
Other general and administrative
Data processing
Rent and occupancy
Professional fees
Depreciation and amortization
Interest expense
Total Expenses		(59,000)
Earnings of equity method investment in related party		(39,000)
Income before provision for income taxes		(39,000)
Provision for income taxes		(11,000)
Net income / (loss)		(28,000)
Total Combined Siebert (unaudited) [Member]
Revenue
Commissions and fees		2,268,000
Margin interest, marketing and distribution fees		3,556,000
Principal transactions		1,890,000
Interest income		1,173,000
Market making		563,000
Stock borrow / stock loan		581,000
Advisory fees		168,000
Other income		79,000
Total Revenue		10,278,000
Expenses
Employee compensation and benefits		4,528,000
Clearing fees, including execution costs		802,000
Technology and communications		422,000
Other general and administrative		733,000
Data processing		543,000
Rent and occupancy		531,000
Professional fees		883,000
Depreciation and amortization		194,000
Interest expense		21,000
Total Expenses		8,657,000
Earnings of equity method investment in related party
Income before provision for income taxes		1,621,000
Provision for income taxes		397,000
Net income / (loss)		$ 1,224,000
Net income per share of common stock
Basic and diluted		$ 0.04
Weighted average shares outstanding
Pro forma shares used to compute net income per share		30,459,804