No definition available.

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

- Definition

As of the balance sheet date, the projected compensation cost in remaining period of current year for equity-based awards made to employees under equity-based compensation awards that have yet to vest.

+ References

No definition available.

+ Details

Name:	gbl_EmployeeServiceShareBasedCompensationNonvestedAwardsProjectedCompensationCostCurrentYear
Namespace Prefix:	gbl_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

As of the balance sheet date, the projected compensation cost next year of equity-based awards made to employees under equity-based compensation awards that have yet to vest.

+ References

No definition available.

+ Details

Name:	gbl_EmployeeServiceShareBasedCompensationNonvestedAwardsProjectedCompensationCostNextFiscalYear
Namespace Prefix:	gbl_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

As of the balance sheet date, the projected compensation cost in eighth fiscal year of equity-based awards made to employees under equity-based compensation awards that have yet to vest.

+ References

No definition available.

+ Details

Name:	gbl_EmployeeServiceShareBasedCompensationNonvestedAwardsProjectedCompensationCostYearEight
Namespace Prefix:	gbl_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

As of the balance sheet date, the projected compensation cost in fifth fiscal year of equity-based awards made to employees under equity-based compensation awards that have yet to vest.

+ References

No definition available.

+ Details

Name:	gbl_EmployeeServiceShareBasedCompensationNonvestedAwardsProjectedCompensationCostYearFive
Namespace Prefix:	gbl_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

As of the balance sheet date, the projected compensation cost in fourth fiscal year of equity-based awards made to employees under equity-based compensation awards that have yet to vest.

+ References

No definition available.

+ Details

Name:	gbl_EmployeeServiceShareBasedCompensationNonvestedAwardsProjectedCompensationCostYearFour
Namespace Prefix:	gbl_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

As of the balance sheet date, the projected compensation cost in seventh fiscal year of equity-based awards made to employees under equity-based compensation awards that have yet to vest.

+ References

No definition available.

+ Details

Name:	gbl_EmployeeServiceShareBasedCompensationNonvestedAwardsProjectedCompensationCostYearSeven
Namespace Prefix:	gbl_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

As of the balance sheet date, the projected compensation cost in sixth fiscal year of equity-based awards made to employees under equity-based compensation awards that have yet to vest.

+ References

No definition available.

+ Details

Name:	gbl_EmployeeServiceShareBasedCompensationNonvestedAwardsProjectedCompensationCostYearSix
Namespace Prefix:	gbl_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

As of the balance sheet date, the projected compensation cost in third fiscal year of equity-based awards made to employees under equity-based compensation awards that have yet to vest.

+ References

No definition available.

+ Details

Name:	gbl_EmployeeServiceShareBasedCompensationNonvestedAwardsProjectedCompensationCostYearThree
Namespace Prefix:	gbl_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

As of the balance sheet date, the projected compensation cost in second fiscal year of equity-based awards made to employees under equity-based compensation awards that have yet to vest.

+ References

No definition available.

+ Details

Name:	gbl_EmployeeServiceShareBasedCompensationNonvestedAwardsProjectedCompensationCostYearTwo
Namespace Prefix:	gbl_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Number of stock award and incentive plan approved by shareholders to provide incentive to key individuals.

+ References

No definition available.

+ Details

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

- Definition

Number of votes per share entitled to class of shares.

+ References

No definition available.

+ Details

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

- References

No definition available.

+ Details

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

- References

No definition available.

+ Details

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

- Definition

Aggregate number of common shares reserved for future issuance.

+ 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 210.5-02.29)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_CommonStockCapitalSharesReservedForFutureIssuance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- References

No definition available.

+ Details

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

- Definition

Unrecognized cost of unvested options awarded to employees as compensation.

+ References

No definition available.

+ Details

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

- Definition

The aggregate amount of noncash, equity-based employee remuneration. This may include the value of stock or unit options, amortization of restricted stock or units, and adjustment for officers' compensation. As noncash, this element is an add back when calculating net cash generated by operating activities using the indirect method.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=56944662&loc=d3e3602-108585

+ Details

Name:	us-gaap_ShareBasedCompensation
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)(i)-(ii)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

The weighted average fair value of nonvested awards on equity-based plans excluding option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, revenue or profit achievement stock award plan) for which the employer is contingently obligated to issue equity instruments or transfer assets to an employee who has not yet satisfied service or performance criteria necessary to gain title to proceeds from the sale of the award or underlying shares or units.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)(i)-(ii)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

+ Details

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

- Definition

Percentage of vesting of share-based compensation awards.

+ References

+ Details

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

- Definition

Period from grant date that an equity-based award expires, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

+ Details

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

- Definition

Number of share options (or share units) exercised during the current period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21463-112644

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-04)
-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)(iv)(2)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.28,29)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 02
-Paragraph 29, 30
-Article 5

+ Details

Name:	us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_AwardTypeAxis=us-gaap_EmployeeStockOptionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardTypeAxis=us-gaap_RestrictedStockMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VestingAxis=us-gaap_ShareBasedCompensationAwardTrancheOneMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VestingAxis=us-gaap_ShareBasedCompensationAwardTrancheTwoMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VestingAxis=us-gaap_ShareBasedCompensationAwardTrancheThreeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VestingAxis=gbl_ShareBasedCompensationAwardTrancheFourMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VestingAxis=gbl_ShareBasedCompensationAwardTrancheFiveMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VestingAxis=gbl_ShareBasedCompensationAwardTrancheSixMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VestingAxis=gbl_ShareBasedCompensationAwardTrancheSevenMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VestingAxis=gbl_ShareBasedCompensationAwardTrancheEightMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VestingAxis=gbl_ShareBasedCompensationAwardTrancheNineMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementScenarioAxis=gbl_FirstQuarterMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementScenarioAxis=gbl_SecondQuarterMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementScenarioAxis=gbl_ThirdQuarterMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementScenarioAxis=gbl_FourthQuarterMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

Stockholders' Equity, Voting Rights, Stock Award and Incentive Plan (Details) $ / shares in Units, $ in Thousands	3 Months Ended					12 Months Ended
	Mar. 31, 2016 USD ($) VoteperShare Plan $ / shares shares	Dec. 31, 2015 USD ($) $ / shares shares	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($) $ / shares shares	Dec. 31, 2015 USD ($) $ / shares shares
Stock Award and Incentive Plan [Abstract]
Number of incentive plans \| Plan	2
Actual and projected stock based compensation expense for RSA shares and options [Abstract]
Actual stock based compensation expense	$ 1,037	$ 4,945	$ 1,644	$ 1,640	$ 1,639	$ 9,868
2016	3,570
2017	2,492
2018	1,685
2019	1,348
2020	737
2021	497
2022	300
2023	127
2024	19
Employee Service Share-based Compensation, Aggregate Disclosures [Abstract]
Compensation cost related to non-vested options not yet recognized	$ 9,700
Stock options exercised (in shares) \| shares	0				0
Stock Options [Member] \| Maximum [Member]
Stock Award and Incentive Plan [Abstract]
Term of nonqualified stock options	10 years
Restricted Stock Awards [Member]
Stock Award and Incentive Plan [Abstract]
RSA shares outstanding (in shares) \| shares	553,100	553,100			707,050	553,100
Average weighted grant price (in dollars per share) \| $ / shares	$ 64.02	$ 64.02			$ 67.43	$ 64.02
Restricted Stock Awards [Member] \| Vesting in Three Years from Date of Grant [Member]
Stock Award and Incentive Plan [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 Award and Incentive Plan [Abstract]
Award vesting percentage	70.00%
Award vesting period	5 years
Restricted Stock Awards [Member] \| Vesting in Year Four from Date of Grant [Member]
Stock Award and Incentive Plan [Abstract]
Award vesting percentage	10.00%
Restricted Stock Awards [Member] \| Vesting in Year Five from Date of Grant [Member]
Stock Award and Incentive Plan [Abstract]
Award vesting percentage	10.00%
Restricted Stock Awards [Member] \| Vesting in Year Six from Date of Grant [Member]
Stock Award and Incentive Plan [Abstract]
Award vesting percentage	10.00%
Restricted Stock Awards [Member] \| Vesting in Year Seven from Date of Grant [Member]
Stock Award and Incentive Plan [Abstract]
Award vesting percentage	10.00%
Restricted Stock Awards [Member] \| Vesting in Year Eight from Date of Grant [Member]
Stock Award and Incentive Plan [Abstract]
Award vesting percentage	10.00%
Restricted Stock Awards [Member] \| Vesting in Year Nine from Date of Grant [Member]
Stock Award and Incentive Plan [Abstract]
Award vesting percentage	10.00%
Restricted Stock Awards [Member] \| Vesting in Year Ten from Date of Grant [Member]
Stock Award and Incentive Plan [Abstract]
Award vesting percentage	10.00%
Q1 [Member]
Actual and projected stock based compensation expense for RSA shares and options [Abstract]
2017	$ 685
2018	464
2019	365
2020	219
2021	145
2022	93
2023	48
2024	7
Q2 [Member]
Actual and projected stock based compensation expense for RSA shares and options [Abstract]
2016	1,036
2017	685
2018	453
2019	365
2020	204
2021	145
2022	93
2023	48
2024	7
Q3 [Member]
Actual and projected stock based compensation expense for RSA shares and options [Abstract]
2016	806
2017	602
2018	402
2019	323
2020	169
2021	114
2022	66
2023	24
2024	5
Q4 [Member]
Actual and projected stock based compensation expense for RSA shares and options [Abstract]
2016	691
2017	520
2018	366
2019	295
2020	145
2021	93
2022	48
2023	7
2024	$ 0
Class A [Member]
Voting Rights [Abstract]
Number of votes per share \| VoteperShare	1
Class A [Member] \| Maximum [Member]
Stock Award and Incentive Plan [Abstract]
Number of shares reserved for issuance under each plan (in shares) \| shares	3,500,000
Class B [Member]
Voting Rights [Abstract]
Number of votes per share \| VoteperShare	10