Debt instrument interest additional interest above eurodollar rate.

No definition available.

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- Definition

Debt instrument interest additional interest above federal fund rate.

+ References

No definition available.

+ Details

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- Definition

Debt Instrument, Issue Price, Percent

+ References

No definition available.

+ Details

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- Definition

Debt Instrument, Number of Debt Agreement Amendments

+ References

No definition available.

+ Details

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- Definition

Debt Instrument, Redemption Price, Percentage of Principal Amount That May Be Redeemed

+ References

No definition available.

+ Details

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- Definition

Line of credit facility, additional borrowing capacity.

+ References

No definition available.

+ Details

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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
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- Definition

Maximum applicable margin rate.

+ References

No definition available.

+ Details

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Balance Type:	na
Period Type:	instant

- Definition

Minimum applicable margin rate.

+ References

No definition available.

+ Details

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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

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Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

+ 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(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

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Balance Type:	credit
Period Type:	instant

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

+ Details

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- 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

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Data Type:	xbrli:stringItemType
Balance Type:	na
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- Definition

Percentage price of original principal amount of debt at which debt can be redeemed by the issuer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 470
-Section 50
-Paragraph 3
-Subparagraph e
-URI http://asc.fasb.org/extlink&oid=75038535&loc=d3e64711-112823

+ Details

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Balance Type:	na
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- 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

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Balance Type:	na
Period Type:	duration

- Definition

Period the derivative contract is outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

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Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Fair value of all derivatives designated as interest rate fair value hedging instruments.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- 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

+ Details

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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

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The cash outflow for a borrowing having initial term of repayment within one year or the normal operating cycle, if longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 15
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3291-108585

+ Details

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Period Type:	duration

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- Details

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- Details

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- Details

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- Details

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Debt (Details)				3 Months Ended		9 Months Ended	12 Months Ended
Debt (Details)	Apr. 05, 2017 USD ($)	Mar. 30, 2017 USD ($)	Mar. 20, 2017 installment	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Sep. 30, 2017 USD ($) amendment	Dec. 31, 2012 USD ($)	Dec. 31, 2016 USD ($)
Bank Term Loan - A Facility
Debt Instrument [Line Items]
Long-term debt, gross				$ 1,447,875,000		$ 1,447,875,000		$ 585,000,000
Bank Term Loan - B Facility
Debt Instrument [Line Items]
Long-term debt, gross				497,500,000		497,500,000		0
Senior Notes
Debt Instrument [Line Items]
Proceeds from issuance of debt		$ 800,000,000
Long-term debt, gross		$ 800,000,000		800,000,000		800,000,000		0
Debt instrument term		8 years
Debt instrument, fixed interest rate		5.125%
Connecticut Economic Development Program
Debt Instrument [Line Items]
Long-term debt, gross				2,500,000		2,500,000	$ 2,500,000.0	2,500,000
Debt instrument term							10 years
Debt instrument, fixed interest rate							3.00%
Revolving credit facility
Debt Instrument [Line Items]
Long-term debt, gross				640,000,000		$ 640,000,000		$ 115,000,000
Interest rate swaps
Debt Instrument [Line Items]
Derivative, term of contract						30 days
Interest rate, fair value hedge derivative at fair value, net (negative fair value)				8,000,000		$ 8,000,000
2016 Agreement
Debt Instrument [Line Items]
Number of debt agreement amendments \| amendment						3
Debt instrument, interest, additional interest above eurodollar rate			1.00%
2016 Agreement \| Federal Funds Rate
Debt Instrument [Line Items]
Debt instrument interest additional interest above federal fund rate			0.50%
Minimum applicable margin rate			0.125%
Maximum applicable margin rate			1.50%
2016 Agreement \| Euro Dollar rate
Debt Instrument [Line Items]
Minimum applicable margin rate			1.125%
Maximum applicable margin rate			2.50%
2016 Agreement \| Term loans
Debt Instrument [Line Items]
Line of credit facility, frequency of payments, number of quarterly installments \| installment			16
2016 Agreement \| Term loans \| Bank Term Loan - A Facility
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity				1,500,000,000.0		$ 1,500,000,000.0
2016 Agreement \| Revolving credit facility
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity				1,200,000,000.0		1,200,000,000.0
CEB
Debt Instrument [Line Items]
Proceeds from issuance of debt	$ 2,780,000,000
CEB \| Senior Notes
Debt Instrument [Line Items]
Proceeds from issuance of debt	800,000,000	$ 800,000,000
Debt instrument, fixed interest rate		5.125%
Debt instrument, issue price, percent		100.00%
Debt instrument, redemption, prince, percentage of principal amount that may be redeemed		40.00%
CEB \| Revolving credit facility
Debt Instrument [Line Items]
Proceeds from issuance of debt	275,000,000
CEB \| Debt instrument, redemption with the proceeds of certain equity offerings \| Senior Notes
Debt Instrument [Line Items]
Debt instrument, redemption price, percentage		105.125%
CEB \| Debt instrument, redemption excluding make whole premium \| Senior Notes
Debt Instrument [Line Items]
Debt instrument, redemption price, percentage		100.00%
CEB \| Debt instrument, redemption due to specific kinds of change of control \| Senior Notes
Debt Instrument [Line Items]
Debt instrument, redemption price, percentage		101.00%
CEB \| 2016 Agreement
Debt Instrument [Line Items]
Proceeds from issuance of debt	1,675,000,000
CEB \| 2016 Agreement \| Term loans \| Bank Term Loan - A Facility
Debt Instrument [Line Items]
Proceeds from issuance of debt	900,000,000
Line of credit facility, additional borrowing capacity	900,000,000
CEB \| 2016 Agreement \| Term loans \| Bank Term Loan - B Facility
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity	500,000,000			500,000,000		$ 500,000,000
Proceeds from issuance of debt	$ 500,000,000
CEB \| Bank Term Loan - B Facility \| Term loans \| LIBOR
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate	2.00%
CEB \| Bank Term Loan - B Facility \| Term loans \| Base Rate
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate	1.00%
Bridge loan \| CEB
Debt Instrument [Line Items]
Proceeds from issuance of debt	$ 300,000,000
Debt instrument term	364 days				364 days
Repayments of short-term debt				$ 200,000,000	$ 100,000,000