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

Face value of debentures repurchased.

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The period when the debt instrument is scheduled to be fully repaid, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

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

- Definition

Refers to the number of equal installments of payments for a debt instrument.

+ References

No definition available.

+ Details

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Namespace Prefix:	gbl_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

The dollars per share dividends declared related to the issuance of debt.

+ References

No definition available.

+ Details

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Namespace Prefix:	gbl_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

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

+ References

No definition available.

+ Details

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

- Definition

Represents the number of debentures repurchased during the period.

+ References

No definition available.

+ Details

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

- Definition

The face value of each debenture issued.

+ References

No definition available.

+ Details

Name:	gbl_ParValueOfDebentures
Namespace Prefix:	gbl_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

+ Details

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

- 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=34725769&loc=d3e28878-108400

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-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28551-108399

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.

+ 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))
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-SubTopic 30
-Section 45
-Paragraph 2
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Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

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

+ References

+ 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

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of the required periodic payments including both interest and principal payments.

+ 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)
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-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=6479336&loc=d3e64711-112823

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

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

+ References

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-Name Accounting Standards Codification
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-SubTopic 470
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-Paragraph 3
-Subparagraph e
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+ Details

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

- Definition

Face (par) amount of the original debt instrument that was repurchased.

+ References

No definition available.

+ Details

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

- 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

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

- Definition

Date the holder must own the stock to be entitled to the dividend, in CCYY-MM-DD format.

+ References

No definition available.

+ Details

Name:	us-gaap_DividendsPayableDateOfRecordDayMonthAndYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

Difference between the fair value of payments made and the carrying amount of debt which is extinguished prior to maturity.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 50
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-Paragraph 2
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-SubTopic 50
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-Paragraph 4
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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Carrying amount of long-term debt, net of unamortized discount or premium, including current and noncurrent amounts. 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 942
-SubTopic 210
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-Paragraph 1
-Subparagraph (SX 210.9-03.16)
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-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 02
-Paragraph 20, 22
-Article 5

+ 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

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The cash outflow during the period from the repayment of aggregate short-term and long-term debt. Excludes payment of capital lease obligations.

+ References

No definition available.

+ Details

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

- Details

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- Details

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- Details

Name:	us-gaap_VariableRateAxis=us-gaap_LondonInterbankOfferedRateLIBORMember
Namespace Prefix:
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- Details

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Debt (Details) $ / shares in Units, $ in Thousands						3 Months Ended
Debt (Details) $ / shares in Units, $ in Thousands	Mar. 18, 2016 USD ($)	Nov. 30, 2015 USD ($) Installment	Nov. 18, 2015 USD ($)	May. 31, 2011 USD ($)	Dec. 31, 2010 USD ($) $ / shares	Mar. 31, 2016 USD ($)	Mar. 31, 2015 USD ($) Debenture	Dec. 31, 2015 USD ($)	May. 04, 2015 USD ($)
Long-term debt [Abstract]
Carrying value						$ 294,103	$ 109,334	$ 309,097
Fair value						$ 293,085	$ 119,981	309,437
Dividend, date of record						Mar. 15, 2016	Mar. 17, 2015
Loss on extinguishment of debt						$ 0	$ (156)
AC 4% PIK Note [Member]
Long-term debt [Abstract]
Carrying value						250,000	0	250,000
Fair value						$ 249,183	0	250,000
Face value of debt		$ 250,000
Debt instrument, maturity date						Nov. 30, 2020
Debt instrument, interest rate		4.00%				4.00%
Debt instrument, number of installments \| Installment		5
Debt instrument, periodic payment		$ 50,000
Loan from GGCP [Member]
Long-term debt [Abstract]
Carrying value						$ 20,000	0	35,000
Fair value						20,161	0	35,000
Face value of debt						$ 35,000
Debt instrument, maturity date						Dec. 28, 2016
Repayment of debt	$ 15,000
Debt instrument, term						1 year
Loan from GGCP [Member] \| 90-day LIBOR [Member]
Long-term debt [Abstract]
Debt instrument, basis spread on variable rate						3.25%
5.875% Senior Notes [Member]
Long-term debt [Abstract]
Carrying value						$ 24,103	99,398	24,097
Fair value						$ 23,741	109,541	24,437
Face value of debt			$ 100,000	$ 100,000
Debt instrument, maturity date						Jun. 01, 2021
Debt instrument, interest rate				5.875%		5.875%
Debt redemption price			101.00%	101.00%
Debt instrument, repurchased face amount			$ 75,800
Debt maturity period						10 years
0% Subordinated Debentures [Member]
Long-term debt [Abstract]
Carrying value						$ 0	9,936	0
Fair value						$ 0	$ 10,440	$ 0
Face value of debt					$ 86,400
Debt instrument, maturity date						Dec. 31, 2015
Debt instrument, interest rate					0.00%	0.00%
Debt redemption price					100.00%
Effective interest rate					7.45%
Debt maturity period						5 years
Dividend, date of record						Dec. 15, 2010
Dividends declared related to issuance of debt (in dollars per share) \| $ / shares					$ 3.20
Par value of debentures (in dollars per share) \| $ / shares					$ 100
Number of debentures repurchased \| Debenture							25,957
Face value of repurchased debentures							$ 2,600
Loss on extinguishment of debt							$ (156)
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