Fair value of stock granted during the period.

No definition available.

Name:	gtlt_FairValueOfStockGranted
Namespace Prefix:	gtlt_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Share-Based Compensation Arrangement By Share-Based Payment Award, Equity Instruments Other Than Options, Forfeited

+ References

No definition available.

+ Details

Name:	gtlt_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeited
Namespace Prefix:	gtlt_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercise Price, Percent

+ References

No definition available.

+ Details

Name:	gtlt_SharebasedCompensationArrangementbySharebasedPaymentAwardOptionsExercisePricePercent
Namespace Prefix:	gtlt_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Share-based Compensation Arrangement by Share-based Payment Award, Outstanding and Issued, Number1

+ References

No definition available.

+ Details

Name:	gtlt_SharebasedCompensationArrangementbySharebasedPaymentAwardOutstandingandIssuedNumber1
Namespace Prefix:	gtlt_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Share-based Compensation Arrangement by Share-based Payment Award, Percent of Awards Vesting After Initial Year

+ References

No definition available.

+ Details

Name:	gtlt_SharebasedCompensationArrangementbySharebasedPaymentAwardPercentofAwardsVestingAfterInitialYear
Namespace Prefix:	gtlt_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5047-113901

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 14.F)
-URI http://asc.fasb.org/extlink&oid=115993241&loc=d3e301413-122809

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (h)(1)(i)
-URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901

+ Details

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

- Definition

Amount of cost not yet recognized for nonvested award under share-based payment arrangement.

+ 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 (i)
-URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901

+ Details

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

- Definition

Amount of cost to be recognized for nonvested award under share-based payment arrangement. Excludes share and unit options.

+ References

+ Details

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

- 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

Number of shares authorized for issuance under share-based payment arrangement.

+ 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)(3)
-URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901

+ Details

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

- Definition

The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable.

+ 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
-URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant
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

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=116856206&loc=d3e5070-113901

+ 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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901

+ Details

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

- Definition

Weighted average remaining contractual term for fully vested and expected to vest options outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. 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=116856206&loc=d3e5070-113901

+ Details

Name:	us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageRemainingContractualTerm1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Purchase price of common stock expressed as a percentage of its fair value.

+ References

No definition available.

+ Details

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

- Details

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

- Details

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

- Details

Name:	us-gaap_OptionIndexedToIssuersEquityTypeAxis=gtlt_StockOption25Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_OptionIndexedToIssuersEquityTypeAxis=gtlt_StockOption75Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_OptionIndexedToIssuersEquityTypeAxis=gtlt_EmployeeDirectorConsultantStockPlanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PlanNameAxis=gtlt_A2014PerformanceAwardsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PlanNameAxis=gtlt_A2015PerformanceAwardsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PlanNameAxis=gtlt_A2017PerformanceSharesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PlanNameAxis=gtlt_A2018PerformanceSharesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PlanNameAxis=gtlt_EmployeeStockPurchasePlanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

SHARE-BASED COMPENSATION - Narrative (Details) - USD ($)	3 Months Ended	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized (in shares)	14,250,000	14,250,000
Number of shares issued (in shares)	11,055,975	11,055,975
Compensation cost not yet recognized	$ 58,200,000	$ 58,200,000
Time-based restricted stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Allocated share-based compensation expense		9,200,000
Fair value of stock granted		29,100,000	$ 43,800,000	$ 31,300,000
Unrecognized compensation cost	55,400,000	55,400,000
Performance-based restricted stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation cost	2,700,000	$ 2,700,000
Stock option 25
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options, vested and expected to vest, outstanding, weighted average remaining contractual term		4 years
Stock option 25 \| Time-based stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options, vested and expected to vest, outstanding, weighted average remaining contractual term		4 years
Stock option 75
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options, vested and expected to vest, outstanding, weighted average remaining contractual term		3 years
Percent of awards vesting after initial year		75.00%
Stock option 75 \| Time-based stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options, vested and expected to vest, outstanding, weighted average remaining contractual term		3 years
Percent of awards vesting after initial year		75.00%
Periodic vesting
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options, exercise price, percent (at least)		100.00%
Periodic vesting \| Employee director consultant stock plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award expiration period		10 years
Periodic vesting \| Stock option 25
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting rights, percentage		25.00%
Periodic vesting \| Stock option 25 \| Time-based stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting rights, percentage		25.00%
2014 Performance awards \| Performance-based restricted stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Fair value of stock granted					$ 8,500,000
2015 Performance awards \| Performance-based restricted stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Allocated share-based compensation expense		$ 1,200,000	7,800,000	6,300,000
Fair value of stock granted				17,400,000
2017 Performance Shares \| Time-based restricted stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Fair value of stock granted			32,600,000
2017 Performance Shares \| Performance-based restricted stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation cost not yet recognized	12,300,000	12,300,000
Allocated share-based compensation expense		7,200,000	$ 5,400,000	$ 0
Amount of unvested shares forfeited	5,100,000	5,100,000
Unrecognized compensation cost	14,900,000	14,900,000
2018 Performance Shares \| Performance-based restricted stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation cost not yet recognized	25,200,000	25,200,000
Allocated share-based compensation expense	0
Fair value of stock granted		31,200,000
Amount of unvested shares forfeited	$ 6,000,000.0	$ 6,000,000.0
Employee stock purchase plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Purchase price of common stock, percent		85.00%
Number of shares available for grant (in shares)	292,897	292,897
Minimum \| Performance-based restricted stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options, vested and expected to vest, outstanding, weighted average remaining contractual term		1 year
Maximum \| Performance-based restricted stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options, vested and expected to vest, outstanding, weighted average remaining contractual term		2 years