Debt (Detail) (USD $)	3 Months Ended	6 Months Ended			3 Months Ended			3 Months Ended			3 Months Ended		7 Months Ended	12 Months Ended	3 Months Ended
	Jun. 30, 2013	Jun. 30, 2013	Mar. 07, 2013	Dec. 31, 2012	Mar. 31, 2013 Refinancing of debt	Jun. 30, 2013 Euro Dollar rate	Jun. 30, 2013 Euro Dollar rate Federal Funds rate	Jun. 30, 2013 Term loans	Mar. 07, 2013 Term loans	Dec. 31, 2012 Term loans	Jun. 30, 2013 Revolver loans	Dec. 31, 2012 Revolver loans	Jun. 30, 2013 Connecticut Economic Development Program	Dec. 31, 2012 Connecticut Economic Development Program	Mar. 31, 2013 Revolving credit facility	Mar. 07, 2013 Revolving credit facility	Jun. 30, 2013 Revolving credit facility Letter of credit
Debt Instrument [Line Items]
Debt instruments, maturity duration		5 years
Line of credit facility, maximum borrowing capacity									$ 150,000,000					$ 5,000,000		$ 600,000,000	$ 40,000,000.0
Line of credit facility, additional borrowing capacity			250,000,000
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		205,000,000				148,125,000	150,000,000	150,000,000	51,875,000	50,000,000				50,000,000
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								0.27%
Debt instrument, interest, additional interest above base rate								1.38%
Derivative, fixed interest rate	2.26%	2.26%
Notional amount of interest rate derivatives	200,000,000	200,000,000
Debt instrument, interest rate at period end	3.64%	3.64%
Line of credit facility, remaining borrowing capacity	544,900,000	544,900,000
Line of credit facility, decrease, forgiveness														2,500,000
Debt instrument, forgiveness period under the terms of the loan													5 years
Interest rate,fair value hedge derivative at fair value, net	7,700,000	7,700,000		10,000,000
Letters of credit outstanding, amount	$ 10,000,000	$ 10,000,000