Percentage of Stock Award Subject to Vesting

No definition available.

Name:	gbli_PercentageOfStockAwardSubjectToVesting
Namespace Prefix:	gbli_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Share based compensation arrangement by share based payment award, award vesting date.

+ References

No definition available.

+ Details

Name:	gbli_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingDate
Namespace Prefix:	gbli_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

Period over which grantee's right to exercise award under share-based payment arrangement is no longer contingent on satisfaction of service or performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, combination of market, performance or service condition.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=120381028&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 grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)(iii)(1)
-URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901

+ Details

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

- Definition

The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)(iii)(1)
-URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)(iii)(2)
-URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901

+ Details

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

- Definition

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.

+ References

No definition available.

+ Details

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

- Definition

Gross number of share options (or share units) granted during the period.

+ References

No definition available.

+ Details

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

- Definition

Number of fully vested and expected to vest options outstanding that can be converted into shares under option plan. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (e)(1)
-URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901

+ Details

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

- Definition

Percentage of vesting of award under share-based payment arrangement.

+ References

+ Details

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

- Definition

Number of non-vested options forfeited.

+ References

No definition available.

+ Details

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

- Details

Name:	srt_TitleOfIndividualAxis=gbli_KeyEmployeesMember
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=gbli_VestingScheduleTwoMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_TitleOfIndividualAxis=gbli_NonEmployeeDirectorMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

Share-Based Compensation Plans - Additional Information (Detail) - $ / shares	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options awarded	0	0	0	0
Unvested stock options forfeited	0	0	0	0
Restricted Stock \| Key Employees
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares granted	0	0	0	36,180
Weighted average fair value per share				$ 35.82
Shares vested				9,063
Stock options, vested and expected to vest		27,117		27,117
Restricted Stock \| Key Employees \| Share-based Payment Arrangement, Tranche 1
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of shares vested on each anniversary of the grant date				16.50%
Vesting date				Jan. 01, 2020
Restricted Stock \| Key Employees \| Share-based Compensation Award, Tranche 2
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of shares vested on each anniversary of the grant date				16.50%
Vesting date				Jan. 01, 2021
Restricted Stock \| Key Employees \| Share-based Compensation Award, Tranche 3
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of shares vested on each anniversary of the grant date				17.00%
Vesting date				Jan. 01, 2022
Restricted Stock \| Key Employees \| Vesting Schedule Two
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of shares vested on each anniversary of the grant date				100.00%
Vesting date				Mar. 15, 2022
Percentage of stock award subject to vesting				50.00%
Restricted Stock \| Non Employee Director
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares granted	29,893	19,275	81,502	50,661
Weighted average fair value per share	$ 22.58	$ 24.97	$ 24.85	$ 28.50
Restricted Stock Units \| Key Employees
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares granted	0	0	161,238	175,498
Weighted average fair value per share			$ 30.32	$ 30.18
Shares vested			3,375
Vesting year			3 years
Restricted Stock Units \| Key Employees \| Share-based Payment Arrangement, Tranche 1
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of shares vested on each anniversary of the grant date			10.00%	10.00%
Vesting date			Jun. 18, 2021	Jun. 18, 2021
Restricted Stock Units \| Key Employees \| Share-based Compensation Award, Tranche 2
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of shares vested on each anniversary of the grant date			20.00%	20.00%
Vesting date			Jun. 18, 2022	Jun. 18, 2022
Restricted Stock Units \| Key Employees \| Share-based Compensation Award, Tranche 3
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of shares vested on each anniversary of the grant date			30.00%	30.00%
Vesting date			Jun. 18, 2023	Jun. 18, 2023
Restricted Stock Units \| Key Employees \| Vesting Schedule Two
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options, vested and expected to vest	90,906		90,906
Percentage of shares vested on each anniversary of the grant date			100.00%
Vesting date			Mar. 15, 2023
Percentage of stock award subject to vesting			50.00%
Restricted Stock Units \| Key Employees \| Vesting Schedule One
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options, vested and expected to vest	66,957		66,957
Restricted Stock Units \| Key Employees \| Share-based Compensation Award, Tranche 4
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of shares vested on each anniversary of the grant date			40.00%	40.00%
Vesting date			Jun. 18, 2024	Jun. 18, 2024
Restricted Stock Units \| Key Employees \| Share-based Payment Arrangement, Tranche 5
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of shares vested on each anniversary of the grant date			16.50%
Vesting date			Jan. 01, 2021
Restricted Stock Units \| Key Employees \| Share-based Compensation Award, Tranche 6
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of shares vested on each anniversary of the grant date			16.50%
Vesting date			Jan. 01, 2022
Restricted Stock Units \| Key Employees \| Share-based Compensation Award, Tranche 7
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of shares vested on each anniversary of the grant date			17.00%
Vesting date			Jan. 01, 2023
Restricted Stock Units \| Non Employee Director
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares granted	41,667		41,667
Vesting year			3 years
Restricted Stock Units \| Non Employee Director \| Share-based Payment Arrangement, Tranche 1
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting date			Jan. 01, 2022
Restricted Stock Units \| Non Employee Director \| Share-based Compensation Award, Tranche 2
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting date			Jan. 01, 2023
Restricted Stock Units \| Non Employee Director \| Share-based Compensation Award, Tranche 3
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting date			Jan. 01, 2024