Debt instrument interest additional interest above eurodollar rate.

No definition available.

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Namespace Prefix:	it_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Debt instrument interest additional interest above federal fund rate.

+ References

No definition available.

+ Details

Name:	it_DebtInstrumentInterestAdditionalInterestAboveFederalFundRate
Namespace Prefix:	it_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Line of credit facility, additional borrowing capacity.

+ References

No definition available.

+ Details

Name:	it_LineofCreditFacilityAdditionalBorrowingCapacity
Namespace Prefix:	it_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Line of Credit Facility, Frequency of Payments, Number of Quarterly Installments

+ References

No definition available.

+ Details

Name:	it_LineofCreditFacilityFrequencyofPaymentsNumberofQuarterlyInstallments
Namespace Prefix:	it_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Maximum applicable margin rate.

+ References

No definition available.

+ Details

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

- Definition

Minimum applicable margin rate.

+ References

No definition available.

+ Details

Name:	it_MinimumApplicableMarginRate
Namespace Prefix:	it_
Data Type:	num:percentItemType
Balance Type:	na
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

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
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Data Type:	xbrli:stringItemType
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=6877327&loc=d3e13212-122682

+ Details

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

- Definition

The cash inflow during the period from additional borrowings in aggregate debt. Includes proceeds from short-term and long-term debt.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 14
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3255-108585

+ Details

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

- Details

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Period Type:

- Details

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Period Type:

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_CreditFacilityAxis=us-gaap_SecuredDebtMember
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Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=it_BankTermLoanMember
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Period Type:

- Details

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Balance Type:
Period Type:

- Details

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- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_SeniorNotesMember
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- Details

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- Details

Name:	us-gaap_VariableRateAxis=us-gaap_LondonInterbankOfferedRateLIBORMember
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Balance Type:
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- Details

Name:	us-gaap_VariableRateAxis=us-gaap_BaseRateMember
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- Details

Name:	us-gaap_LongtermDebtTypeAxis=it_BankTermLoanBFacilityMember
Namespace Prefix:
Data Type:	na
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- Details

Name:	us-gaap_ShortTermDebtTypeAxis=us-gaap_BridgeLoanMember
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Debt - 2016 Credit Agreement (Details)	Apr. 05, 2017 USD ($) installment	Mar. 30, 2017 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
2016 Credit Facility Agreement
Debt Instrument [Line Items]
Debt instrument, interest, additional interest above eurodollar rate	1.00%
2016 Credit Facility Agreement \| Federal Funds rate
Debt Instrument [Line Items]
Minimum applicable margin rate	0.125%
Maximum applicable margin rate	1.50%
2016 Credit Facility Agreement \| Euro Dollar rate
Debt Instrument [Line Items]
Minimum applicable margin rate	1.125%
Maximum applicable margin rate	2.50%
Federal Funds rate
Debt Instrument [Line Items]
Additional interest above federal fund rate	0.50%
Secured Debt \| 2016 Credit Facility Agreement
Debt Instrument [Line Items]
Line of credit facility, frequency of payments in quarterly installments \| installment	16
Secured Debt \| Term loan \| 2016 Credit Facility Agreement
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity			$ 1,500,000,000
Revolving Credit Facility \| 2016 Credit Facility Agreement
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity			1,200,000,000
CEB
Debt Instrument [Line Items]
Proceeds from issuance of debt	$ 2,800,000,000
CEB \| 2016 Credit Facility Agreement
Debt Instrument [Line Items]
Proceeds from issuance of debt	$ 1,675,000,000
CEB \| Senior notes
Debt Instrument [Line Items]
Proceeds from issuance of debt		$ 800,000,000.0
CEB \| Secured Debt \| Term B facility \| LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate	2.00%
CEB \| Secured Debt \| Term B facility \| Base Rate
Debt Instrument [Line Items]
Basis spread on variable rate	1.00%
CEB \| Secured Debt \| Term loan \| 2016 Credit Facility Agreement
Debt Instrument [Line Items]
Additional borrowing capacity	$ 900,000,000.0
CEB \| Secured Debt \| Term B facility \| 2016 Credit Facility Agreement
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity			$ 500,000,000.0	$ 496,300,000
Proceeds from issuance of debt	500,000,000.0
CEB \| Revolving Credit Facility
Debt Instrument [Line Items]
Proceeds from issuance of debt	275,000,000.0
Bridge Loan \| CEB
Debt Instrument [Line Items]
Proceeds from issuance of debt	$ 300,000,000.0
Debt instrument term	364 days