The amount available for debt and equity issuance under Shelf Registration.

No definition available.

Name:	gbl_AmountAvailableForDebtAndEquityIssuanceUnderShelfRegistration
Namespace Prefix:	gbl_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The number of shares issuable in exchange for the original debt being converted in a noncash (or part noncash) transaction.

+ References

No definition available.

+ Details

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Namespace Prefix:	gbl_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

The cash outflow for the extinguishment of long-term borrowing before its maturity.

+ References

No definition available.

+ Details

Name:	gbl_EarlyRepaymentOfDebt
Namespace Prefix:	gbl_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The maximum amount of debt and equity to be issued under the Shelf Registration.

+ References

No definition available.

+ Details

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Namespace Prefix:	gbl_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Period of interest included in Initial Deposit, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

Name:	gbl_PeriodOfInterestIncludedInInitialDeposit
Namespace Prefix:	gbl_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

The price per share of the conversion feature embedded in the debt instrument.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 5
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=6928298&loc=SL6031898-161870

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

- Definition

Face (par) amount of debt instrument at time of issuance.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=64930739&loc=d3e28878-108400

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28551-108399

+ Details

Name:	us-gaap_DebtInstrumentFaceAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(2))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage price of original principal amount of debt at which debt can be redeemed by the issuer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 470
-Section 50
-Paragraph 3
-Subparagraph e
-URI http://asc.fasb.org/extlink&oid=6479336&loc=d3e64711-112823

+ Details

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Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Period of time between issuance and maturity of debt instrument, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentTerm
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount, after accumulated amortization, of debt issuance costs. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28555-108399

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.17)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_DeferredFinanceCostsNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

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

+ References

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=66011266&loc=d3e33918-111571

+ Details

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

- Definition

Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(16))
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-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_LongTermDebt
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- References

No definition available.

+ Details

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Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission.

+ References

No definition available.

+ Details

Name:	us-gaap_LongTermDebtFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=gbl_ConvertibleNoteDueAugust2021Member
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Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

Name:	us-gaap_DebtInstrumentAxis=gbl_ACPaidInKindNoteDueNovember2020Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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Data Type:	na
Balance Type:
Period Type:

- Details

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Period Type:

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Debt (Details) - USD ($) $ / shares in Units, $ in Thousands, shares in Millions		3 Months Ended	6 Months Ended
	May 31, 2011	Jun. 30, 2017	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016	Aug. 15, 2016	Nov. 30, 2015	May 04, 2015
Long-term debt [Abstract]
Carrying value		$ 213,985	$ 213,985	$ 274,109	$ 233,955
Prepayment of debt			20,000	0
Maximum amount of debt and equity to be issued under shelf registration								$ 500,000
Amount available for debt and equity issuance under shelf registration		500,000	500,000
Level 2 [Member]
Long-term debt [Abstract]
Fair value		228,597	228,597	276,392	237,013
4.5% Convertible Notes [Member]
Long-term debt [Abstract]
Carrying value		$ 109,853	$ 109,853	$ 0	$ 109,835
Debt instrument, interest rate		4.50%	4.50%	4.50%	4.50%	4.50%
Debt instrument, term			5 years
Face value of debt						$ 110,000
Debt instrument, maturity date			Aug. 15, 2021
Debt issuance costs		$ 147	$ 147	$ 0	$ 165	$ 174
4.5% Convertible Notes [Member] \| Common Class A [Member]
Long-term debt [Abstract]
Debt instrument, conversion price (in dollars per share)						$ 55.00
Debt instrument, shares issuable in conversion (in shares)						2
4.5% Convertible Notes [Member] \| Level 2 [Member]
Long-term debt [Abstract]
Fair value		$ 112,176	112,176	0	111,525
4.5% Convertible Notes [Member] \| GGCP Holdings LLC [Member]
Long-term debt [Abstract]
Equity method investment, ownership percentage						63.00%
Period of interest included in initial deposit		6 months
AC 4% PIK Note [Member]
Long-term debt [Abstract]
Carrying value		$ 80,000	$ 80,000	250,000	100,000
Debt instrument, interest rate		4.00%	4.00%				4.00%
Face value of debt		$ 80,000	$ 80,000				$ 250,000
Debt instrument, maturity date			Nov. 30, 2020
Prepayment of debt		10,000	$ 20,000
AC 4% PIK Note [Member] \| Principal Amount Due on November 30, 2018 [Member]
Long-term debt [Abstract]
Face value of debt		10,000	10,000
AC 4% PIK Note [Member] \| Principal Amount Due on November 30, 2019 [Member]
Long-term debt [Abstract]
Face value of debt		20,000	20,000
AC 4% PIK Note [Member] \| Principal Amount Due on November 30, 2020 [Member]
Long-term debt [Abstract]
Face value of debt		50,000	50,000
AC 4% PIK Note [Member] \| Level 2 [Member]
Long-term debt [Abstract]
Fair value		91,599	91,599	251,909	100,930
5.875% Senior Notes [Member]
Long-term debt [Abstract]
Carrying value		$ 24,132	$ 24,132	$ 24,109	$ 24,120
Debt instrument, interest rate	5.875%	5.875%	5.875%	5.875%	5.875%
Debt instrument, term			10 years
Face value of debt	$ 100,000
Debt instrument, maturity date			Jun. 01, 2021
Debt issuance costs		$ 93	$ 93	$ 116	$ 105
Debt redemption price	101.00%
5.875% Senior Notes [Member] \| Level 2 [Member]
Long-term debt [Abstract]
Fair value		$ 24,822	$ 24,822	$ 24,483	$ 24,558