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Business and Significant Accounting Policies (Detail) (USD $)
 12 Months Ended
Dec. 31, 2014
segment
Dec. 31, 2013
Dec. 31, 2012
Business and Significant Accounting Policies [Line Items]      
Long-term line of credit $ 405,000,000us-gaap_LineOfCredit [1] $ 205,000,000us-gaap_LineOfCredit [1]  
Number of reportable segments 3us-gaap_NumberOfReportableSegments    
Unbilled fees receivable 44,000,000us-gaap_UnbilledReceivablesCurrent 37,000,000us-gaap_UnbilledReceivablesCurrent  
Stock-based compensation expense 38,845,000us-gaap_ShareBasedCompensation 34,735,000us-gaap_ShareBasedCompensation 36,378,000us-gaap_ShareBasedCompensation
Rent expense 31,500,000us-gaap_OperatingLeasesRentExpenseNet 30,800,000us-gaap_OperatingLeasesRentExpenseNet 30,300,000us-gaap_OperatingLeasesRentExpenseNet
Depreciation expense 31,186,000us-gaap_Depreciation 28,996,000us-gaap_Depreciation 25,369,000us-gaap_Depreciation
Net capitalized development costs for internal use software   14,100,000us-gaap_CapitalizedComputerSoftwareNet  
Amortization of capitalized internal software development costs 8,200,000us-gaap_CapitalizedComputerSoftwareAmortization 8,200,000us-gaap_CapitalizedComputerSoftwareAmortization 7,400,000us-gaap_CapitalizedComputerSoftwareAmortization
Amortization of intangibles 8,226,000us-gaap_AmortizationOfIntangibleAssets 5,446,000us-gaap_AmortizationOfIntangibleAssets 4,402,000us-gaap_AmortizationOfIntangibleAssets
Pension expense 3,400,000us-gaap_PensionExpense 3,800,000us-gaap_PensionExpense 2,600,000us-gaap_PensionExpense
Outstanding debt   205,000,000us-gaap_DebtCurrent  
Foreign currency transaction gain (loss) (1,700,000)us-gaap_ForeignCurrencyTransactionGainLossRealized (900,000)us-gaap_ForeignCurrencyTransactionGainLossRealized (2,300,000)us-gaap_ForeignCurrencyTransactionGainLossRealized
Derivative instruments, gain (loss) recognized in income, net 4,000us-gaap_DerivativeGainLossOnDerivativeNet 4,000us-gaap_DerivativeGainLossOnDerivativeNet 3,000us-gaap_DerivativeGainLossOnDerivativeNet
Cost of repurchased stock 419,712,000us-gaap_StockRepurchasedDuringPeriodValue 196,696,000us-gaap_StockRepurchasedDuringPeriodValue 111,304,000us-gaap_StockRepurchasedDuringPeriodValue
Payments for purchases of treasury stock 432,006,000us-gaap_PaymentsForRepurchaseOfCommonStock 181,736,000us-gaap_PaymentsForRepurchaseOfCommonStock 111,304,000us-gaap_PaymentsForRepurchaseOfCommonStock
Foreign currency exchange contracts      
Business and Significant Accounting Policies [Line Items]      
Derivative instruments, gain (loss) recognized in income, net $ 600,000us-gaap_DerivativeGainLossOnDerivativeNet
/ us-gaap_FinancialInstrumentAxis
= it_ForeignCurrencyExchangeContractsMember		 $ (100,000)us-gaap_DerivativeGainLossOnDerivativeNet
/ us-gaap_FinancialInstrumentAxis
= it_ForeignCurrencyExchangeContractsMember		 $ 600,000us-gaap_DerivativeGainLossOnDerivativeNet
/ us-gaap_FinancialInstrumentAxis
= it_ForeignCurrencyExchangeContractsMember
Software      
Business and Significant Accounting Policies [Line Items]      
Remaining amortization period 3 years    
Minimum | Trade Names      
Business and Significant Accounting Policies [Line Items]      
Remaining amortization period 2 years    
Minimum | Customer Relationships      
Business and Significant Accounting Policies [Line Items]      
Remaining amortization period 4 years    
Minimum | Content      
Business and Significant Accounting Policies [Line Items]      
Remaining amortization period 1 year 6 months    
Minimum | Noncompete Agreements      
Business and Significant Accounting Policies [Line Items]      
Remaining amortization period 4 years    
Maximum | Trade Names      
Business and Significant Accounting Policies [Line Items]      
Remaining amortization period 5 years    
Maximum | Customer Relationships      
Business and Significant Accounting Policies [Line Items]      
Remaining amortization period 7 years    
Maximum | Content      
Business and Significant Accounting Policies [Line Items]      
Remaining amortization period 4 years    
Maximum | Noncompete Agreements      
Business and Significant Accounting Policies [Line Items]      
Remaining amortization period 5 years    
[1] As of December 31, 2014, $20.0 million of debt was classified as short term and $385.0 million was classified as long term on the Consolidated Balance Sheets.