Stock-Based Compensation (Detail) - Weighted-Average Assumptions Used To Determine Fair Value Of SARs Grants On Date Of Grant Using Black-Scholes-Merton Valuation Model (Stock Appreciation Rights (SARs) [Member])	9 Months Ended
	Sep. 30, 2012		Sep. 30, 2011
Stock Appreciation Rights (SARs) [Member]
Expected dividend yield (2)	0.00%	[1],[2]	0.00%	[1],[2]
Expected stock price volatility (3)	40.00%	[1],[3]	38.00%	[1],[3]
Risk-free interest rate (4)	0.80%	[1],[4]	2.20%	[1],[4]
Expected life in years (5)	4 years 219 days	[1],[5]	4 years 292 days	[1],[5]

[1] The Company did not make any SARs grants during the three months ended September 30, 2012 and 2011.

[2] The dividend yield assumption is based on the history and expectation of the Company's dividend payouts. Historically, Gartner has not paid cash dividends on its Common Stock.

[3] The determination of expected stock price volatility was based on both historical Common Stock prices and implied volatility from publicly traded options in the Common Stock.

[4] The risk-free interest rate is based on the yield of a U.S. Treasury security with a maturity similar to the expected life of the award.

[5] The expected life represents the Company's weighted-average estimate of the period of time the SARs are expected to be outstanding (that is, the period between the service inception date and the expected exercise date). Beginning January 1, 2012, the expected life has been calculated based on the Company's historical exercise data. Previously, the Company determined the expected life based on a simplified calculation permitted by SEC SAB No. 107 and SAB No. 110 since the necessary historical exercise data was not available. The change in methodology had an insignificant impact on the calculation of the expected life.