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Significant Accounting Policies (Details)	12 Months Ended
Significant Accounting Policies (Details)	Dec. 31, 2019 USD ($) Segment	Dec. 31, 2018 USD ($)	May 31, 2011
Investments in Securities [Abstract]
Impairment on investment in debt securities, classified as held-to-maturity	$ 0	$ 0
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Portion of revenue derived from advisory and management fees	89.00%	89.00%
Institutional and private wealth management incentive fees receivable	$ 0	$ 0
Management fees receivable on closed-end preferred shares	5,800,000	0
Goodwill and Identifiable Intangible Assets [Abstract]
Impairment on intangible assets	$ 0	0
Management Fee [Abstract]
Management fee expense percentage	10.00%
Management fee allocated to other employees	$ 1,500,000	900,000
Business Segment [Abstract]
Number of operating segments \| Segment	1
New Accounting Pronouncement [Abstract]
Increase to other assets	$ 2,037,000	$ (234,000)
5.875% Senior Notes [Member]
New Accounting Pronouncement [Abstract]
Debt instrument, interest rate	5.875%	5.875%	5.875%
Debt issuance cost	$ 34,000	$ 57,000
Restricted Stock Awards [Member] \| Vesting in Three Years from Date of Grant [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting percentage	30.00%
Award vesting period	3 years
Restricted Stock Awards [Member] \| Vesting in Five Years from Date of Grant [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting percentage	70.00%
Award vesting period	5 years
Stock Options [Member] \| Vesting in Three Years from Date of Grant [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting percentage	75.00%
Award vesting period	3 years
Stock Options [Member] \| Vesting in Four Years from Date of Grant [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting percentage	25.00%
Award vesting period	4 years
Mr. Gabelli [Member] \| Award Granted for 2016 [Member] \| Restricted Stock Awards [Member] \| Vesting on January 1, 2020 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting percentage	100.00%
Mr. Gabelli [Member] \| Award Granted for 2016 [Member] \| Restricted Stock Awards [Member] \| Vesting in 2018 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting percentage		25.00%
Mr. Gabelli [Member] \| Award Granted for FH 2017 [Member] \| Restricted Stock Awards [Member] \| Vesting on July 1, 2018 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting percentage	100.00%
Mr. Gabelli [Member] \| Award Granted for Q4 2017 [Member] \| Restricted Stock Awards [Member] \| Vesting in April 1, 2019 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting percentage	100.00%
Fixed Assets Other Than Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Fixed assets with net book value	$ 325,000	$ 362,000
Accumulated Depreciation	$ 2,900,000	2,800,000
Fixed Assets Other Than Leasehold Improvements [Member] \| Minimum [Member]
Property, Plant and Equipment [Line Items]
Estimated useful life of assets	4 years
Fixed Assets Other Than Leasehold Improvements [Member] \| Maximum [Member]
Property, Plant and Equipment [Line Items]
Estimated useful life of assets	7 years
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Fixed assets with net book value	$ 1,100,000	1,300,000
Depreciation and amortization	1,200,000	562,000
Estimated annual depreciation and amortization expense	$ 1,200,000
Period of estimate for future depreciation and amortization	3 years
The GDL Fund [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Performance fee receivable	$ 2,200,000	0
Gabelli Merger Plus+ Trust [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Performance fee receivable	$ 0	$ 0
GAMCO Merger Arbitrage SICAV [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Incentive fee earned as percentage of gross return	20.00%	20.00%
Performance fee receivable	$ 5,200,000	$ 1,300,000
Open End Class AAA Shares of the Funds [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Fee percentages paid to distributors based on fund performance	0.25%
TETON Westwood Funds [Member] \| Class A [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Fee percentages paid to distributors based on fund performance	0.50%
TETON Westwood Intermediate Bond Fund [Member] \| Class A [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Fee percentages paid to distributors based on fund performance	0.35%
Class A Shares of Gabelli Enterprise Mergers and Acquisitions Fund [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Fee percentages paid to distributors based on fund performance	0.45%
Class C Shares [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Fee percentages paid to distributors based on fund performance	1.00%
ASU 2016-02 [Member]
New Accounting Pronouncement [Abstract]
Impact of ASU adoption on retained earnings	$ (106,000)
Increase to other assets	650,000
Increase to lease liability obligations	$ 756,000
Reclassification pursuant to adoption of ASU 2016-01, net of tax		(106,000)
ASU 2016-02 [Member] \| Additional Paid-in Capital [Member]
New Accounting Pronouncement [Abstract]
Reclassification pursuant to adoption of ASU 2016-01, net of tax		0
ASU 2016-02 [Member] \| Retained Earnings (Deficit) [Member]
New Accounting Pronouncement [Abstract]
Reclassification pursuant to adoption of ASU 2016-01, net of tax		$ (106,000)