Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2021	Oct. 31, 2021
Document and Entity Information
Document Type	10-Q
Document Quarterly Report	true
Document Transition Report	false
Document Period End Date	Sep. 30, 2021
Entity File Number	000-51829
Entity Registrant Name	COGENT COMMUNICATIONS HOLDINGS, INC.
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	46-5706863
Entity Address, Address Line One	2450 N Street N.W.
Entity Address, Country	US
Entity Address, City or Town	Washington, D.C
Entity Address, Postal Zip Code	20037
City Area Code	202
Local Phone Number	295-4200
Title of 12(b) Security	Common Stock
Trading Symbol	CCOI
Security Exchange Name	NASDAQ
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		47,673,776
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q3
Entity Central Index Key	0001158324
Amendment Flag	false