Significant Accounting Policies (Details)	12 Months Ended
Dec. 31, 2020 USD ($) Segment	Dec. 31, 2019 USD ($)
Investments in Securities [Abstract]
Impairment on investment in debt securities, classified as held-to-maturity	$ 0	$ 0
Major Revenue-Generating Services and Revenue Recognition [Abstract]
Portion of revenue derived from advisory and management fees	90.00%	89.00%
Institutional and private wealth management incentive fees receivable	$ 0	$ 0
Management fees receivable on closed-end preferred shares	$ 2,500,000	5,800,000
Management Fee [Abstract]
Management fee expense percentage	10.00%
Management fee allocated to other employees	$ 800,000	1,500,000
Business Segment [Abstract]
Number of operating segments \| Segment	1
New Accounting Pronouncement [Abstract]
Impact of ASU adoption on retained earnings	$ 394,386,000	362,515,000
Increase to other assets	1,736,000	2,037,000
Fixed Assets Other Than Leasehold Improvements [Member]
Depreciation and Amortization [Abstract]
Fixed assets with net book value	2,500,000	325,000
Accumulated Depreciation	$ 2,900,000	2,900,000
Fixed Assets Other Than Leasehold Improvements [Member] \| Minimum [Member]
Depreciation and Amortization [Abstract]
Estimated useful life of assets	4 years
Fixed Assets Other Than Leasehold Improvements [Member] \| Maximum [Member]
Depreciation and Amortization [Abstract]
Estimated useful life of assets	7 years
Leasehold Improvements [Member]
Depreciation and Amortization [Abstract]
Fixed assets with net book value	$ 800,000	1,100,000
Depreciation and amortization	900,000	1,200,000
Estimated annual depreciation and amortization expense	$ 900,000
Period of estimate for future depreciation and amortization	3 years
The GDL Fund [Member]
Major Revenue-Generating Services and Revenue Recognition [Abstract]
Performance fee receivable	$ 100,000	2,200,000
Gabelli Merger Plus+ Trust [Member]
Major Revenue-Generating Services and Revenue Recognition [Abstract]
Performance fee receivable	$ 0	$ 0
GAMCO Merger Arbitrage SICAV [Member]
Major Revenue-Generating Services and Revenue Recognition [Abstract]
Incentive fee earned as percentage of gross return	20.00%	20.00%
Performance fee receivable	$ 5,600,000	$ 5,200,000
Open End Class AAA Shares of the Funds [Member]
Major Revenue-Generating Services and Revenue Recognition [Abstract]
Fee percentages paid to distributors based on fund performance	0.25%
Class C Shares [Member]
Major Revenue-Generating Services and Revenue Recognition [Abstract]
Fee percentages paid to distributors based on fund performance	1.00%
Restricted Stock Awards [Member] \| Vesting in Three Years from Date of Grant [Member]
Stock Based Compensation [Abstract]
Award vesting percentage	30.00%
Award vesting period	3 years
Restricted Stock Awards [Member] \| Vesting in Five Years from Date of Grant [Member]
Stock Based Compensation [Abstract]
Award vesting percentage	70.00%
Award vesting period	5 years
Stock Options [Member] \| Vesting in Three Years from Date of Grant [Member]
Stock Based Compensation [Abstract]
Award vesting percentage	75.00%
Award vesting period	3 years
Stock Options [Member] \| Vesting in Four Years from Date of Grant [Member]
Stock Based Compensation [Abstract]
Award vesting percentage	25.00%
Award vesting period	4 years
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

The percentage per year for distribution or service fees.

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Incentive fee earned as percentage of gross return of the fund.

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Mr. Gabelli has the ability to allocate as portion of management fees to his designee, this element represents the amounts allocated to other recipients.

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Management fee expense is incentive-based and entirely variable compensation in the amount of this percentage of the aggregate pre-tax profits which is paid to Mr. Gabelli or his designee for acting as CEO pursuant to his 2008 Employment Agreement so long as he is an executive of GBL and devotes the substantial majority of his working time to the business.

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The amount of receivable related to management fees receivable on closed-end preferred shares during the period.

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The Company derived a percentage of its total revenues from advisory and management fees, including incentive fees.

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Amount of accumulated depreciation, depletion and amortization for physical assets used in the normal conduct of business to produce goods and services.

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Amount of increase in lease obligation from new lease.

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The aggregate expense recognized in the current period that allocates the cost of tangible assets, intangible assets, or depleting assets to periods that benefit from use of the assets.

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Amount of increase (decrease) in operating assets classified as other.

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Number of operating segments. An operating segment is a component of an enterprise: (a) that engages in business activities from which it may earn revenues and incur expenses (including revenues and expenses relating to transactions with other components of the same enterprise), (b) whose operating results are regularly reviewed by the enterprise's chief operating decision maker to make decisions about resources to be allocated to the segment and assess its performance, and (c) for which discrete financial information is available. An operating segment may engage in business activities for which it has yet to earn revenues, for example, start-up operations may be operating segments before earning revenues.

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Amount, before tax, of other-than-temporary impairment (OTTI) on investment in debt security measured at amortized cost (held-to-maturity), recognized in earnings and other comprehensive income (OCI).

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Amount after accumulated depreciation, depletion and amortization of physical assets used in the normal conduct of business to produce goods and services and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, and furniture and fixtures.

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Useful life of long lived, physical assets used in the normal conduct of business and not intended for resale, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Examples include, but not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment.

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The cumulative amount of the reporting entity's undistributed earnings or deficit.

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