Base fee percentage.

No definition available.

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

- Definition

Employee Stock Purchase Plan Maximum Number of Shares that May be Purchase by Eligible Participants

+ References

No definition available.

+ Details

Name:	it_EmployeeStockPurchasePlanMaximumNumberofSharesthatMaybePurchasebyEligibleParticipants
Namespace Prefix:	it_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Share Based Compensation Arrangement By Employee Stock Purchase Plan Stock Purchases as Percentage Of Employee Compensation Maximum.

+ References

No definition available.

+ Details

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

- Definition

Share based Compensation Arrangement by Share based Payment Award, Equity Instruments Other than Options, Granted in Period, Weighted Average Remaining Contractual Term.

+ References

No definition available.

+ Details

Name:	it_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantedInPeriodWeightedAverageRemainingContractualTerm
Namespace Prefix:	it_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Share Based Compensation Arrangement By Share Based Payment Award Exercisable Price Percentage Of Closing Price Of Another Class Of Stock.

+ References

No definition available.

+ Details

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

- Definition

Share based compensation arrangement by share based payment award expiry period from date of grant.

+ References

No definition available.

+ Details

Name:	it_ShareBasedCompensationArrangementByShareBasedPaymentAwardExpiryPeriodFromDateOfGrant
Namespace Prefix:	it_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Share Based Compensation Arrangement By Share Based Payment Award Maximum Share Value Authorized For Purchase Under Employee Stock Purchase Plan.

+ References

No definition available.

+ Details

Name:	it_ShareBasedCompensationArrangementByShareBasedPaymentAwardMaximumShareValueAuthorizedForPurchaseUnderEmployeeStockPurchasePlan
Namespace Prefix:	it_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Share Based Compensation Arrangement by Share Based Payment Award, Equity Instruments Other Than Options, Outstanding, Intrinsic Value.

+ References

No definition available.

+ Details

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

- Definition

Share based Compensation Arrangement by Share based Payment Award, Equity Instruments Other Than Options, Vested and Expected to Vest, Exercisable, Aggregate Intrinsic Value.

+ References

No definition available.

+ Details

Name:	it_SharebasedCompensationArrangementbySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedandExpectedtoVestExercisableAggregateIntrinsicValue.
Namespace Prefix:	it_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Face amount or stated value per share of common stock.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(29))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_CommonStockParOrStatedValuePerShare
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

- Definition

The amount of net income (loss) for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04(19))
-URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=64863997&loc=d3e1252-109256

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03(21))
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04(23))
-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882

+ Details

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

- Definition

Unrecognized cost of unvested share-based compensation awards.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (i)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

+ Details

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

- Definition

Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

+ Details

Name:	us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of cash inflow (outflow) from financing activities, including discontinued operations. Financing activity cash flows include obtaining resources from owners and providing them with a return on, and a return of, their investment; borrowing money and repaying amounts borrowed, or settling the obligation; and obtaining and paying for other resources obtained from creditors on long-term credit.

+ References

+ Details

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

- Definition

Amount of cash inflow (outflow) from operating activities, including discontinued operations. Operating activity cash flows include transactions, adjustments, and changes in value not defined as investing or financing activities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 24
-URI http://asc.fasb.org/extlink&oid=66902209&loc=d3e3521-108585

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-URI http://asc.fasb.org/extlink&oid=66902209&loc=d3e3602-108585

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 25
-URI http://asc.fasb.org/extlink&oid=66902209&loc=d3e3536-108585

+ Details

Name:	us-gaap_NetCashProvidedByUsedInOperatingActivities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	duration

- Definition

The cash inflow associated with the amount received from the stock plan during the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 14
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=66902209&loc=d3e3255-108585

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Financing Activities
-URI http://asc.fasb.org/extlink&oid=6513228

+ Details

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

- Definition

The aggregate amount of noncash, equity-based employee remuneration. This may include the value of stock or unit options, amortization of restricted stock or units, and adjustment for officers' compensation. As noncash, this element is an add back when calculating net cash generated by operating activities using the indirect method.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=66902209&loc=d3e3602-108585

+ Details

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

- Definition

Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=6415400&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://www.xbrl.org/2003/role/presentationRef
-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=6415400&loc=d3e5070-113901

+ Details

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

- Definition

The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)(i)-(ii)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

+ Details

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

- Definition

Weighted average remaining contractual term for equity-based awards excluding options, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (e)(1)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsOutstandingWeightedAverageRemainingContractualTerms
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
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

The maximum number of shares (or other type of equity) originally approved (usually by shareholders and board of directors), net of any subsequent amendments and adjustments, for awards under the equity-based compensation plan. As stock or unit options and equity instruments other than options are awarded to participants, the shares or units remain authorized and become reserved for issuance under outstanding awards (not necessarily vested).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)(3)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

+ Details

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

- Details

Name:	us-gaap_TitleOfIndividualAxis=us-gaap_ExecutiveOfficerMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	it_RestrictedEquityAwardTypeAxis=it_ServiceBasedAwardsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

Name:	us-gaap_TitleOfIndividualAxis=it_NonExecutiveMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PlanNameAxis=it_UnvestedMember
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:	it_RestrictedEquityAwardTypeAxis=us-gaap_PerformanceSharesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

Name:	us-gaap_AdjustmentsForNewAccountingPronouncementsAxis=it_AccountingStandardsUpdate201609Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_NewAccountingPronouncementEarlyAdoptionAxis=us-gaap_NewAccountingPronouncementEarlyAdoptionEffectMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AdjustmentsForNewAccountingPronouncementsAxis=it_AccountingStandardsUpdate201609StatutoryTaxWithholdingComponentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Stock-Based Compensation (Detail) - USD ($)	12 Months Ended
Stock-Based Compensation (Detail) - USD ($)	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares available for grant	6,200,000
Common stock par value (in Dollars per share)	$ 0.0005	$ 0.0005
Stock-based compensation expense	$ 46,661,000	$ 46,149,000	$ 38,845,000
Total share-based compensation cost not yet recognized	$ 49,400,000
Remaining weighted average service period	2 years 3 months 18 days
Weighted average remaining contractual term	1 year 1 month 6 days
Diluted earnings per share (in Dollars per share)	$ 2.31	$ 2.06	$ 2.03
Operating cash flow	$ 365,632,000	$ 345,561,000	$ 346,779,000
Cash used by financing activities	$ (174,686,000)	(67,690,000)	(208,670,000)
Executive Officer \| Service-based Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grants during period (in shares)	300,000
Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Employee stock purchase plan, stock purchases as a percentage of employee compensation, maximum	10.00%
Maximum share value authorized for purchase under employee stock purchase plan	$ 23,750
Exercisable price percentage of closing price of another class of stock	95.00%
Maximum number of shares that may be purchased by eligible participants	900,000
Proceeds from stock grants	$ 9,300,000	$ 7,500,000	7,800,000
Stock Appreciation Rights (SARs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Remaining weighted average service period	6 years 1 month 10 days
Award vesting period	4 years
Award expiry period from date of grant	7 years
Total compensation cost not yet recognized, period for recognition	4 years
Equity instruments number of shares unvested (in shares)	1,300,000	1,300,000
Aggregate intrinsic value of outstanding shares	$ 46,800,000
Aggregate intrinsic value of shares vested and exercisable	$ 25,100,000
Grants during period (in shares)	400,000
Stock Appreciation Rights (SARs) \| Non Executive
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grants during period (in shares)	300,000
Stock Appreciation Rights (SARs) \| Unvested
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Equity instruments number of shares unvested (in shares)	800,000
Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Equity instruments number of shares unvested (in shares)	1,300,000	1,400,000
Grants during period (in shares)	600,000
Restricted Stock Units (RSUs) \| Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period	4 years
Grants during period (in shares)	600,000
Restricted Stock Units (RSUs) \| Service-based Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period	4 years
Common Stock Equivalents (CSEs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Equity instruments number of shares unvested (in shares)	107,338	105,664
Grants during period (in shares)	7,069
Base fee percentage	50.00%
Retirement Eligible Employees Equity Award
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 19,400,000	$ 20,100,000	$ 14,800,000
Accounting Standards Update 2016-09 \| New Accounting Pronouncement, Early Adoption, Effect
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Diluted earnings per share (in Dollars per share)	$ 0.12
Accounting Standards Update 2016-09, Statutory Tax Withholding Component \| New Accounting Pronouncement, Early Adoption, Effect
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Operating cash flow	$ 10,000,000
Cash used by financing activities	$ (10,000,000)