Debt (Detail) (USD $)	9 Months Ended			3 Months Ended	9 Months Ended			9 Months Ended					9 Months Ended						1 Months Ended	3 Months Ended
	Sep. 30, 2013	Mar. 07, 2013	Dec. 31, 2012	Mar. 31, 2013 Refinancing of debt	Sep. 30, 2013 Federal Funds rate	Sep. 30, 2013 Euro Dollar rate	Sep. 30, 2013 Euro Dollar rate Federal Funds rate	Sep. 30, 2013 Term loans		Mar. 07, 2013 Term loans	Dec. 31, 2012 Term loans		Sep. 30, 2013 Revolver loans		Mar. 07, 2013 Revolver loans	Dec. 31, 2012 Revolver loans		Mar. 07, 2013 Revolver loans Letter of credit	Dec. 31, 2012 Connecticut Economic Development Program	Mar. 31, 2013 Revolving credit facility
Debt Instrument [Line Items]
Debt instruments, maturity duration								5 years
Line of credit facility, maximum borrowing capacity										$ 150,000,000.0					$ 600,000,000.0			$ 40,000,000.0	$ 5,000,000
Line of credit facility, additional borrowing capacity		250,000,000.0
Line of credit facility, frequency of payments								16 consecutive quarterly installments
Debt instrument, maturity date								Mar. 07, 2018					Mar. 07, 2018
Line of credit facility, amount outstanding	205,000,000		205,000,000					146,250,000	[1]	150,000,000	150,000,000	[1]	53,750,000	[1],[2]	50,000,000	50,000,000	[1],[2]
Debt issuance cost				300,000																3,600,000
Line of credit facility, interest rate description	either (i) the greatest of: the Administrative Agent's prime rate; the average rate on overnight federal funds plus 1/2 of 1%; and the Eurodollar rate (adjusted for statutory reserves) plus 1%, in each case plus a margin equal to between 0.25% and 0.75% depending on Gartner's leverage ratio as of the end of the four consecutive fiscal quarters most recently ended, or (ii) the Eurodollar rate (adjusted for statutory reserves) plus a margin equal to between 1.25% and 1.75%, depending on Gartner's leverage ratio as of the end of the four consecutive fiscal quarters most recently ended																		10 year maturity and bears a 3.0% fixed rate of interest
Debt instrument, maturity period																			10 years
Debt instrument, fixed interest rate																			3.00%
Debt instrument, period principal payments are deferred																			5 years
Debt instrument, interest, additional interest above federal fund rate					0.50%
Debt Instrument, interest, additional interest above eurodollar rate	1.00%
Minimum applicable margin rate						1.25%	0.25%
Maximum applicable margin rate						1.75%	0.75%
Debt Instrument, interest rate increase	0.02
Debt instrument, interest base rate								0.24%
Debt instrument, interest, additional interest above base rate								1.38%
Derivative, fixed interest rate	2.26%
Notional amount of interest rate derivatives	200,000,000.0
Debt instrument, interest rate at period end	3.64%
Line of credit facility, remaining borrowing capacity	543,000,000
Line of credit facility, decrease, forgiveness																			2,500,000.0
Debt instrument, forgiveness period under the terms of the loan																			5 years
Interest rate, fair value hedge derivative at fair value, net (negative fair value)	7,300,000		10,000,000
Letters of credit outstanding, amount	$ 9,700,000

[1] Both the term and revolver loan rates consisted of a floating Eurodollar base rate of 0.24% plus a margin of 1.38%. However, the Company has an interest rate swap contract which converts the floating Eurodollar base rate to a 2.26% fixed base rate on the first $200.0 million of Company borrowings (see below). As a result, the Company’s effective annual interest rate on the $200.0 million of outstanding term and revolver debt as of September 30, 2013, including the swap and margin, was 3.64%.

[2] The Company had approximately $543.0 million of available borrowing capacity on the revolver (not including the expansion feature) as of September 30, 2013.