Debt Instrument, Issue Price, Percentage

No definition available.

Name:	it_DebtInstrumentIssuePricePercentage
Namespace Prefix:	it_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Line of Credit Facility, Accordion Feature, Increase Limit

+ References

No definition available.

+ Details

Name:	it_LineOfCreditFacilityAccordionFeatureIncreaseLimit
Namespace Prefix:	it_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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

Name:	us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=119993939&loc=d3e28551-108399

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=114775985&loc=d3e28878-108400

+ Details

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

- Definition

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

+ 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
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

Name:	us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
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_DebtInstrumentLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage of principal amount of debt redeemed.

+ References

No definition available.

+ Details

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

- 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

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

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ 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
-Subparagraph (SX 210.5-02.19(b),22(b))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

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

- Definition

The fee, expressed as a percentage of the line of credit facility, for available but unused credit capacity under the credit facility.

+ References

No definition available.

+ Details

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

- Definition

The cash inflow from a debt initially having maturity due after one year or beyond the operating cycle, if longer.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 14
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3255-108585

+ Details

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

- Details

Name:	us-gaap_DebtInstrumentAxis=it_SeniorNotesDue2030Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_SeniorNotesMember
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Balance Type:
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- Details

Name:	us-gaap_DebtInstrumentAxis=it_SeniorNotesDue2028Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=it_A2020CreditAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	it_VariableRateComponentsAxis=it_VariableRateComponentOneMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableRateAxis=us-gaap_LondonInterbankOfferedRateLIBORMember
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Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableRateAxis=us-gaap_BaseRateMember
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Data Type:	na
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- Details

Name:	srt_RangeAxis=srt_MinimumMember
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- Details

Name:	srt_RangeAxis=srt_MaximumMember
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- Details

Name:	it_VariableRateComponentsAxis=it_VariableRateComponentTwoMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_CreditFacilityAxis=us-gaap_RevolvingCreditFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_CreditFacilityAxis=us-gaap_LetterOfCreditMember
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Debt - Narrative (Details) - USD ($)		3 Months Ended
Debt - Narrative (Details) - USD ($)	Sep. 28, 2020	Mar. 31, 2021	Jun. 22, 2020
Senior Notes Due 2030 \| Senior Notes
Debt Instrument [Line Items]
Aggregate principal amount	$ 800,000,000.0	$ 800,000,000.0
Debt instrument, fixed interest rate	3.75%
Debt instrument, issue price, percent	100.00%
Percentage of principal amount redeemed		40.00%
Senior Notes Due 2028 \| Senior Notes
Debt Instrument [Line Items]
Aggregate principal amount		$ 800,000,000.0	$ 800,000,000.0
Debt instrument, fixed interest rate			4.50%
Debt instrument, issue price, percent			100.00%
Percentage of principal amount redeemed		40.00%
2020 Credit Agreement \| Variable Rate Component, One \| London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Basis spread on variable rate	1.00%
2020 Credit Agreement \| Variable Rate Component, One \| Base Rate \| Minimum
Debt Instrument [Line Items]
Basis spread on variable rate	0.125%
2020 Credit Agreement \| Variable Rate Component, One \| Base Rate \| Maximum
Debt Instrument [Line Items]
Basis spread on variable rate	1.25%
2020 Credit Agreement \| Variable Rate Component, Two \| London Interbank Offered Rate (LIBOR) \| Minimum
Debt Instrument [Line Items]
Basis spread on variable rate	1.125%
2020 Credit Agreement \| Variable Rate Component, Two \| London Interbank Offered Rate (LIBOR) \| Maximum
Debt Instrument [Line Items]
Basis spread on variable rate	2.25%
2020 Credit Agreement \| Term Loan Facility
Debt Instrument [Line Items]
Aggregate principal amount	$ 400,000,000.0
Debt term	5 years
Proceeds from borrowings	$ 400,000,000.0
2020 Credit Agreement \| Revolving credit facility \| Line of Credit
Debt Instrument [Line Items]
Debt term	5 years
Accordion feature, increase limit	$ 1,000,000,000.0
Line of credit facility, maximum borrowing capacity	$ 1,000,000,000.0
2020 Credit Agreement \| Revolving credit facility \| Line of Credit \| Minimum
Debt Instrument [Line Items]
Unused borrowing capacity, fee percent	0.175%
2020 Credit Agreement \| Revolving credit facility \| Line of Credit \| Maximum
Debt Instrument [Line Items]
Unused borrowing capacity, fee percent	0.40%
2020 Credit Agreement \| Letter of Credit \| Line of Credit
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity	$ 75,000,000.0