Acquisitions and Divestiture - Summary of Exited Space Liability (Details) - CEB	12 Months Ended
Dec. 31, 2017 USD ($)
Liability Related to Exited Space [Roll Forward]
Beginning liability balance	$ 0
Charges and adjustments	13,087,000
Payments	(126,000)
Ending liability balance	$ 12,961,000

Business Combination, Separately Recognized Transactions, Charges And Adjustments, Net

No definition available.

Name:	it_BusinessCombinationSeparatelyRecognizedTransactionsChargesAndAdjustmentsNet
Namespace Prefix:	it_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Business Combination Separately Recognized Transactions, Liabilities Recognized [Roll Forward]

+ References

No definition available.

+ Details

Name:	it_BusinessCombinationSeparatelyRecognizedTransactionsLiabilitiesRecognizedRollForward
Namespace Prefix:	it_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The expenses and losses recorded for each transaction with the acquiree that was recognized separately from the acquisition of assets and assumptions of liabilities in the business combination.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (e)(3)
-URI http://asc.fasb.org/extlink&oid=79982066&loc=d3e1392-128463

+ Details

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

- Definition

The amount of liabilities recognized recorded in a transaction with the acquiree for each transaction that is recognized separately from the acquisition of assets and assumptions of liabilities in the business combination.

+ References

+ Details

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

- Details

Name:	us-gaap_BusinessAcquisitionAxis=it_CEBInc.Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: