Subsequent Events	3 Months Ended
Mar. 31, 2017
Subsequent Events [Abstract]
Subsequent Events	On April 5, 2017, the Company completed the previously announced acquisition of CEB. Gartner acquired all of the outstanding shares of CEB in a cash and stock transaction with a total enterprise value of approximately $3.5 billion gross. Gartner paid $2.7 billion in cash at close and issued 7.4 million of its common shares. The Company borrowed approximately $2.8 billion in conjunction with the acquisition, which included $800.0 million of principal amount Senior Notes (see Note 7 — Debt) issued on March 30, 2017; approximately $1.7 billion borrowed on April 5, 2017 under the 2016 Credit Agreement, which was amended for a third time on April 5, 2017; and $300.0 million borrowed under a 364-day Bridge Credit Facility, entered into on April 5, 2017. The Company had $3.6 billion in total debt outstanding after the close of the acquisition. The third amendment to the 2016 Credit Agreement and the 364-day Bridge Credit Facility, and the amounts borrowed under those borrowing arrangements in conjunction with the acquisition, are discussed in more detail below. Third Amendment to the 2016 Credit Agreement On April 5, 2017, in conjunction with the closing of the CEB acquisition, the Company entered into a third amendment to the 2016 Credit Agreement with its lenders (the "Incremental Amendment"). The Incremental Amendment increased the aggregate principal amount of the existing Term loan A facility by $900.0 million and added a new incremental Term loan B facility in an aggregate principal amount of $500.0 million. Immediately upon entry into the Incremental Amendment, the Company drew down $900.0 million under the increased Term loan A facility and $500.0 million under the new Term loan B facility and made an additional draw of $275.0 million on its existing revolving credit facility. Along with the funds raised through the issuance of the Senior Notes and the 364-Bridge Credit Facility, the amounts drawn under the 2016 Credit Agreement were used to fund the cash paid for the CEB acquisition. The additional amount drawn under the Term loan A facility has the same maturity date and is subject to the same interest, repayment terms, amortization schedules, representations and warranties, affirmative and negative covenants and events of default as the amounts outstanding under such facility prior to entry by the Company into the Incremental Amendment. The Term loan B facility contains representations and warranties, affirmative and negative covenants and events of default that are the same as the Term loan A facility and revolving credit facility, except that a breach of financial maintenance covenants will not result in an event of default under the Term loan B facility unless the lenders under the revolving credit facility and Term loan A facility have accelerated the revolving loans and Term loan A loans and terminated their commitments thereunder. Additionally, the Term loan B facility includes mandatory prepayment requirements related to asset sales (subject to reinvestment), debt incurrence (other than permitted debt) and excess cash flow, subject to certain limitations described therein. Any voluntary prepayment of the Term loan B facility made in connection with a repricing transaction in the first six months following April 5, 2017 will be subject to a 1.00% prepayment premium. The Term loan B facility will mature on April 5, 2024 and amounts outstanding thereunder will bear interest at a rate per annum equal to, at the option of Gartner, (i) adjusted LIBOR plus 2.00% or (ii) an alternate base rate plus 1.00%. 364-day Bridge Credit Facility Also on April 5, 2017, and in connection with the closing of the CEB acquisition, the Company entered into a senior unsecured 364-day Bridge Credit Facility in an aggregate principal amount of $300.0 million, which amount was immediately drawn by the Company to fund a portion of the costs associated with the acquisition. The 364-day Bridge Credit Facility will mature on the 364th day after the closing date of the acquisition. Amounts outstanding under the 364-day Bridge Credit Facility will bear interest at a rate per annum equal to, at the option of the Company: (i) adjusted LIBOR plus 2.75% or (ii) an alternate base rate plus 1.75%, with the margins on both increasing by0.25% 180 days after the closing date and an additional 0.25% each 90 days thereafter. The 364-day Bridge Credit Facility contains representations and warranties, affirmative and negative covenants and events of default that are substantially the same as in the 2016 Credit Agreement. Additionally, the 364-Day Bridge Credit Facility includes mandatory prepayment requirements related to the receipt by the Company of repatriated funds from its foreign subsidiaries, subject to certain exceptions and reduced by any taxes payable or reasonably estimated by the Company to be payable upon such repatriation and proceeds from certain debt and equity issuances.

On April 5, 2017, the Company completed the previously announced acquisition of CEB. Gartner acquired all of the outstanding shares of CEB in a cash and stock transaction with a total enterprise value of approximately $3.5 billion gross. Gartner paid $2.7 billion in cash at close and issued 7.4 million of its common shares.

The Company borrowed approximately $2.8 billion in conjunction with the acquisition, which included $800.0 million of principal amount Senior Notes (see Note 7 — Debt) issued on March 30, 2017; approximately $1.7 billion borrowed on April 5, 2017 under the 2016 Credit Agreement, which was amended for a third time on April 5, 2017; and $300.0 million borrowed under a 364-day Bridge Credit Facility, entered into on April 5, 2017. The Company had $3.6 billion in total debt outstanding after the close of the acquisition. The third amendment to the 2016 Credit Agreement and the 364-day Bridge Credit Facility, and the amounts borrowed under those borrowing arrangements in conjunction with the acquisition, are discussed in more detail below.

Third Amendment to the 2016 Credit Agreement

On April 5, 2017, in conjunction with the closing of the CEB acquisition, the Company entered into a third amendment to the 2016 Credit Agreement with its lenders (the "Incremental Amendment"). The Incremental Amendment increased the aggregate principal amount of the existing Term loan A facility by $900.0 million and added a new incremental Term loan B facility in an aggregate principal amount of $500.0 million. Immediately upon entry into the Incremental Amendment, the Company drew down $900.0 million under the increased Term loan A facility and $500.0 million under the new Term loan B facility and made an additional draw of $275.0 million on its existing revolving credit facility. Along with the funds raised through the issuance of the Senior Notes and the 364-Bridge Credit Facility, the amounts drawn under the 2016 Credit Agreement were used to fund the cash paid for the CEB acquisition.

The additional amount drawn under the Term loan A facility has the same maturity date and is subject to the same interest, repayment terms, amortization schedules, representations and warranties, affirmative and negative covenants and events of default as the amounts outstanding under such facility prior to entry by the Company into the Incremental Amendment.

The Term loan B facility contains representations and warranties, affirmative and negative covenants and events of default that are the same as the Term loan A facility and revolving credit facility, except that a breach of financial maintenance covenants will not result in an event of default under the Term loan B facility unless the lenders under the revolving credit facility and Term loan A facility have accelerated the revolving loans and Term loan A loans and terminated their commitments thereunder. Additionally, the Term loan B facility includes mandatory prepayment requirements related to asset sales (subject to reinvestment), debt incurrence (other than permitted debt) and excess cash flow, subject to certain limitations described therein. Any voluntary prepayment of the Term loan B facility made in connection with a repricing transaction in the first six months following April 5, 2017 will be subject to a 1.00% prepayment premium. The Term loan B facility will mature on April 5, 2024 and amounts outstanding thereunder will bear interest at a rate per annum equal to, at the option of Gartner, (i) adjusted LIBOR plus 2.00% or (ii) an alternate base rate plus 1.00%.

364-day Bridge Credit Facility

Also on April 5, 2017, and in connection with the closing of the CEB acquisition, the Company entered into a senior unsecured 364-day Bridge Credit Facility in an aggregate principal amount of $300.0 million, which amount was immediately drawn by the Company to fund a portion of the costs associated with the acquisition. The 364-day Bridge Credit Facility will mature on the 364th day after the closing date of the acquisition. Amounts outstanding under the 364-day Bridge Credit Facility will bear interest at a rate per annum equal to, at the option of the Company: (i) adjusted LIBOR plus 2.75% or (ii) an alternate base rate plus 1.75%, with the margins on both increasing by0.25% 180 days after the closing date and an additional 0.25% each 90 days thereafter. The 364-day Bridge Credit Facility contains representations and warranties, affirmative and negative covenants and events of default that are substantially the same as in the 2016 Credit Agreement. Additionally, the 364-Day Bridge Credit Facility includes mandatory prepayment requirements related to the receipt by the Company of repatriated funds from its foreign subsidiaries, subject to certain exceptions and reduced by any taxes payable or reasonably estimated by the Company to be payable upon such repatriation and proceeds from certain debt and equity issuances.