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Income Taxes (Detail) (USD $)
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Income Tax [Line Items]      
Non-current deferred tax assets $ 17,960,000us-gaap_DeferredTaxAssetsLiabilitiesNetNoncurrent $ 24,371,000us-gaap_DeferredTaxAssetsLiabilitiesNetNoncurrent  
Valuation allowance (570,000)us-gaap_DeferredTaxAssetsValuationAllowance (617,000)us-gaap_DeferredTaxAssetsValuationAllowance  
Unrecognized tax benefits 20,645,000us-gaap_UnrecognizedTaxBenefits 14,488,000us-gaap_UnrecognizedTaxBenefits 17,552,000us-gaap_UnrecognizedTaxBenefits
Unrecognized tax benefits reductions resulting from settlements with taxing authorities and lapse of applicable statute of limitations 4,900,000it_UnrecognizedTaxBenefitsReductionsResultingFromSettlementsWithTaxingAuthoritiesAndLapseOfApplicableStatuteOfLimitations    
Income tax penalties and interest expense 3,300,000us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestExpense 3,300,000us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestExpense  
Income tax examination, penalties and interest expense 100,000us-gaap_IncomeTaxExaminationPenaltiesAndInterestExpense 300,000us-gaap_IncomeTaxExaminationPenaltiesAndInterestExpense  
Unrecognized tax benefits that would impact effective tax rate 16,500,000us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate    
Unrecognized tax benefits that would result adjustments to other tax accounts 4,100,000it_UnrecognizedTaxBenefitsthatWouldResultAdjustmentstoOtherTaxAccounts    
Earnings of certain non US subsidiaries indefinitely reinvested 186,500,000it_EarningsOfCertainNonUSSubsidiariesIndefinitelyReinvested    
Undistributed foreign earnings 35,800,000us-gaap_DeferredTaxLiabilitiesUndistributedForeignEarnings    
Stamford      
Income Tax [Line Items]      
Unrecognized tax benefits 1,300,000us-gaap_UnrecognizedTaxBenefits
/ it_FacilityAxis
= it_StamfordMember		 1,300,000us-gaap_UnrecognizedTaxBenefits
/ it_FacilityAxis
= it_StamfordMember		  
State and local jurisdiction      
Income Tax [Line Items]      
Operating loss carryforwards 7,800,000us-gaap_OperatingLossCarryforwards
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_StateAndLocalJurisdictionMember		    
Tax credit carryforwards 1,600,000us-gaap_TaxCreditCarryforwardAmount
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_StateAndLocalJurisdictionMember		    
State and local jurisdiction | Minimum      
Income Tax [Line Items]      
Tax credit carryforwards, expiration period 2 years    
State and local jurisdiction | Maximum      
Income Tax [Line Items]      
Tax credit carryforwards, expiration period 5 years    
Non US      
Income Tax [Line Items]      
Operating loss carryforwards 30,200,000us-gaap_OperatingLossCarryforwards
/ us-gaap_IncomeTaxAuthorityAxis
= it_NonUSMember		    
Foreign tax credit carryforwards 3,400,000us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsForeign
/ us-gaap_IncomeTaxAuthorityAxis
= it_NonUSMember		    
Expire within one to five years | State and local jurisdiction      
Income Tax [Line Items]      
Operating loss carryforwards 700,000us-gaap_OperatingLossCarryforwards
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_StateAndLocalJurisdictionMember
/ it_PeriodAxis
= it_ExpireWithinOneToFiveYearsMember		    
Expire within one to five years | State and local jurisdiction | Minimum      
Income Tax [Line Items]      
Operating loss carryforwards expiration period 1 year    
Expire within one to five years | State and local jurisdiction | Maximum      
Income Tax [Line Items]      
Operating loss carryforwards expiration period 5 years    
Expire within six to fifteen years | State and local jurisdiction      
Income Tax [Line Items]      
Operating loss carryforwards 6,300,000us-gaap_OperatingLossCarryforwards
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_StateAndLocalJurisdictionMember
/ it_PeriodAxis
= it_ExpireWithinSixToFifteenYearsMember		    
Expire within six to fifteen years | State and local jurisdiction | Minimum      
Income Tax [Line Items]      
Operating loss carryforwards expiration period 6 years    
Expire within six to fifteen years | State and local jurisdiction | Maximum      
Income Tax [Line Items]      
Operating loss carryforwards expiration period 15 years    
Expire within sixteen to twenty years | State and local jurisdiction      
Income Tax [Line Items]      
Operating loss carryforwards 800,000us-gaap_OperatingLossCarryforwards
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_StateAndLocalJurisdictionMember
/ it_PeriodAxis
= it_ExpireWithinSixteenToTwentyYearsMember		    
Expire within sixteen to twenty years | State and local jurisdiction | Minimum      
Income Tax [Line Items]      
Operating loss carryforwards expiration period 16 years    
Expire within sixteen to twenty years | State and local jurisdiction | Maximum      
Income Tax [Line Items]      
Operating loss carryforwards expiration period 20 years    
Expire over next twenty years | State and local jurisdiction      
Income Tax [Line Items]      
Operating loss carryforwards expiration period 20 years    
Expire over next twenty years | Non US      
Income Tax [Line Items]      
Operating loss carryforwards 2,600,000us-gaap_OperatingLossCarryforwards
/ us-gaap_IncomeTaxAuthorityAxis
= it_NonUSMember
/ it_PeriodAxis
= it_ExpireOverNextTwentyYearsMember		    
Carried forward indefinitely | Non US      
Income Tax [Line Items]      
Operating loss carryforwards 27,600,000us-gaap_OperatingLossCarryforwards
/ us-gaap_IncomeTaxAuthorityAxis
= it_NonUSMember
/ it_PeriodAxis
= it_CarriedForwardIndefinitelyMember		    
Prepaid expenses and other current assets      
Income Tax [Line Items]      
Current deferred tax assets 17,500,000us-gaap_DeferredTaxAssetsLiabilitiesNetCurrent
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_PrepaidExpensesAndOtherCurrentAssetsMember		 19,500,000us-gaap_DeferredTaxAssetsLiabilitiesNetCurrent
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_PrepaidExpensesAndOtherCurrentAssetsMember		  
Accounts payable and accrued liabilities      
Income Tax [Line Items]      
Current deferred tax liabilities 2,100,000us-gaap_DeferredTaxLiabilitiesCurrent
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_AccountsPayableAndAccruedLiabilitiesMember		 8,700,000us-gaap_DeferredTaxLiabilitiesCurrent
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_AccountsPayableAndAccruedLiabilitiesMember		  
Other assets      
Income Tax [Line Items]      
Non-current deferred tax assets 18,000,000us-gaap_DeferredTaxAssetsLiabilitiesNetNoncurrent
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherAssetsMember		 24,400,000us-gaap_DeferredTaxAssetsLiabilitiesNetNoncurrent
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherAssetsMember		  
Other liabilities      
Income Tax [Line Items]      
Non-current deferred tax liabilities 600,000us-gaap_DeferredTaxLiabilitiesNoncurrent
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherLiabilitiesMember		 200,000us-gaap_DeferredTaxLiabilitiesNoncurrent
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherLiabilitiesMember		  
Liability for uncertain tax positions $ 15,700,000it_LiabilityForUncertainTaxPositions
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherLiabilitiesMember		 $ 11,400,000it_LiabilityForUncertainTaxPositions
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherLiabilitiesMember