Debt - Narrative (Details) - USD ($)			12 Months Ended
Debt - Narrative (Details) - USD ($)	Mar. 26, 2024	Sep. 28, 2020	Dec. 31, 2025	Nov. 20, 2025	Dec. 31, 2024	Jun. 18, 2021	Jun. 22, 2020
Senior Notes
Debt Instrument [Line Items]
Carrying Amount				$ 799,900,000
2024 Credit Agreement \| Senior Notes
Debt Instrument [Line Items]
Carrying Amount				274,400,000
2031 Notes \| Senior Notes
Debt Instrument [Line Items]
Aggregate principal amount				$ 350,000,000.0
Debt instrument, fixed interest rate			4.95%	4.95%	4.95%
Debt instrument, issue price, percent				99.97%
2035 Notes \| Senior Notes
Debt Instrument [Line Items]
Aggregate principal amount				$ 450,000,000.0
Debt instrument, fixed interest rate			5.60%	5.60%	5.60%
Debt instrument, issue price, percent				99.992%
2029 Notes \| Senior Notes
Debt Instrument [Line Items]
Aggregate principal amount						$ 600,000,000
Debt instrument, fixed interest rate			3.63%		3.63%	3.625%
Debt instrument, issue price, percent						100.00%
2030 Notes \| Senior Notes
Debt Instrument [Line Items]
Aggregate principal amount		$ 800,000,000
Debt instrument, fixed interest rate		3.75%	3.75%		3.75%
Debt instrument, issue price, percent		100.00%
Percentage of principal amount redeemed		40.00%
2028 Notes \| Senior Notes
Debt Instrument [Line Items]
Aggregate principal amount							$ 800,000,000
Debt instrument, fixed interest rate			4.50%		4.50%		4.50%
Debt instrument, issue price, percent							100.00%
2020 Credit Agreement \| Variable Rate Component One
Debt Instrument [Line Items]
Additional interest above base rate		0.50%
Revolving Credit Facility \| 2024 Credit Agreement \| Unsecured Debt
Debt Instrument [Line Items]
Aggregate principal amount	$ 1,000,000,000
Debt instrument term	5 years
Line of credit facility, additional borrowing capacity	$ 750,000,000
Additional interest above base rate			1.35%
Contractual annualized interest rate			5.35%
Debt instrument, fixed interest rate			4.00%
Revolving Credit Facility \| 2024 Credit Agreement \| Unsecured Debt \| Minimum
Debt Instrument [Line Items]
Line of credit facility, commitment fee percentage	0.125%
Revolving Credit Facility \| 2024 Credit Agreement \| Unsecured Debt \| Maximum
Debt Instrument [Line Items]
Line of credit facility, commitment fee percentage	0.25%
Revolving Credit Facility \| 2024 Credit Agreement \| Unsecured Debt \| Variable Rate Component One
Debt Instrument [Line Items]
Additional interest above base rate			0.10%
Revolving Credit Facility \| 2024 Credit Agreement \| Unsecured Debt \| Variable Rate Component Two \| Minimum
Debt Instrument [Line Items]
Additional interest above base rate	1.125%
Revolving Credit Facility \| 2024 Credit Agreement \| Unsecured Debt \| Variable Rate Component Two \| Maximum
Debt Instrument [Line Items]
Additional interest above base rate	1.75%
Letter of Credit \| 2024 Credit Agreement \| Unsecured Debt
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity	$ 75,000,000

GARTNER INC 10-K