Name:	gbl_InvestmentAdvisoryServicesAbstract
Namespace Prefix:	gbl_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

The percentage of rights held of a stockholder to vote on matters of corporate policy and who will make up the board of directors. Voting often involves decisions on issuing securities, initiating corporate actions and making substantial changes in the corporation's operations.

No definition available.

Name:	gbl_OwnershipPercentageOfVotingRights
Namespace Prefix:	gbl_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

Refers to number of shares owned by related party.

No definition available.

Name:	gbl_SharesOwnedByRelatedParty
Namespace Prefix:	gbl_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

he base rate related to the sublease of space in the building

No definition available.

Name:	gbl_SubleaseRentalBaseRate
Namespace Prefix:	gbl_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

The base rate related to the sublease of space in the building.

No definition available.

Name:	gbl_SubleaseRentalBaseRateShares
Namespace Prefix:	gbl_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	duration

This rate is for electricity and taxes related to the subleased space.

No definition available.

Name:	gbl_SubleaseRentalRateForUtilitiesAndTaxesOnSubleaseProperty
Namespace Prefix:	gbl_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	duration

The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=6382943&loc=d3e33918-111571

Name:	us-gaap_EquityMethodInvestmentOwnershipPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

Net rentable area for properties owned.

No definition available.

Name:	us-gaap_NetRentableArea
Namespace Prefix:	us-gaap_
Data Type:	num:areaItemType
Balance Type:	na
Period Type:	instant

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

No definition available.

Name:	us-gaap_RelatedPartyTransactionLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Amount of revenue, fees and commissions earned from transactions between (a) a parent company and its subsidiaries; (b) subsidiaries of a common parent; (c) an entity and trusts for the benefit of employees, for example, but not limited to, pension and profit-sharing trusts that are managed by or under the trusteeship of the entity's management; (d) an entity and its principal, owners, management, or members of their immediate families; and (e) affiliates.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.1(e))
-URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Related Parties
-URI http://asc.fasb.org/extlink&oid=16382449

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 946
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.6-07.1(c))
-URI http://asc.fasb.org/extlink&oid=6488393&loc=d3e606610-122999

Name:	us-gaap_RevenueFromRelatedParties
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=gbl_GgcpHoldingsLlcMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_StatementClassOfStockAxis=us-gaap_CommonClassBMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=us-gaap_CoVenturerMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_StatementClassOfStockAxis=us-gaap_CommonClassAMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=gbl_EntityControlledByMembersOfChairmanSFamilyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=gbl_LictCorporationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=gbl_TetonMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=gbl_MjgAssociatesIncMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	dei_LegalEntityAxis=gbl_GBLMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_IncomeStatementLocationAxis=gbl_AdvisoryFeesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=gbl_ManhattanPartnersILpMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=gbl_SwanPartnersLpMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=gbl_AffiliatedFundsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	dei_LegalEntityAxis=gbl_GDistributorsLlcMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_IncomeStatementLocationAxis=gbl_DistributionFeesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: