Fair value portion of investment in debt security measured at amortized cost (held-to-maturity).

No definition available.

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

- Definition

Fair value portion of debt instrument payable, including, but not limited to, notes payable and loans payable.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 55
-Paragraph 69C
-URI http://asc.fasb.org/extlink&oid=123466577&loc=SL123495737-112612

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-Publisher FASB
-Name Accounting Standards Codification
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+ Details

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

- References

No definition available.

+ Details

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

- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 1B
-Subparagraph (b)
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-Publisher FASB
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-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

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Balance Type:	na
Period Type:	instant

- Definition

Amount, after allowance for credit loss, of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), investment in debt security measured at amortized cost (held-to-maturity), and investment in debt security measured at fair value with change in fair value recognized in net income (trading).

+ References

No definition available.

+ Details

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

- References

No definition available.

+ Details

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

- Definition

Fair value portion of notes payable.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
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+ Details

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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

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Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_SubordinatedDebtMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_SeniorNotesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USTreasurySecuritiesMember
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- Details

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Fair Value, Financial Assets and Liabilities Disclosed But Not Carried at Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020	May 31, 2011
5.875% Senior Notes [Member]
Debt instruments [Abstract]
Debt instrument, interest rate	5.875%	5.875%	5.875%
Carrying Value [Member]
Investments in debt securities [Abstract]
Total	$ 0	$ 64,988
Debt instruments [Abstract]
Total	52,102	24,215
Carrying Value [Member] \| Subordinated Notes [Member]
Debt instruments [Abstract]
Notes payable	52,102	0
Carrying Value [Member] \| 5.875% Senior Notes [Member] \| Senior Notes [Member]
Debt instruments [Abstract]
Notes payable	0	24,215
Carrying Value [Member] \| U.S. Treasury Bills [Member]
Investments in debt securities [Abstract]
Debt securities held-to-maturity	0	64,988
Level 1 [Member] \| Fair Value [Member]
Investments in debt securities [Abstract]
Total	0	64,994
Level 1 [Member] \| Fair Value [Member] \| U.S. Treasury Bills [Member]
Investments in debt securities [Abstract]
Debt securities held-to-maturity	0	64,994
Level 2 [Member] \| Fair Value [Member]
Debt instruments [Abstract]
Total	52,102	24,554
Level 2 [Member] \| Fair Value [Member] \| Subordinated Notes [Member]
Debt instruments [Abstract]
Notes payable	52,102	0
Level 2 [Member] \| Fair Value [Member] \| 5.875% Senior Notes [Member] \| Senior Notes [Member]
Debt instruments [Abstract]
Notes payable	$ 0	$ 24,554