Long-term debt: - Limitations under the Indentures (Details)	9 Months Ended
	Sep. 30, 2025 USD ($)	Apr. 11, 2025	May 02, 2024
Long-term debt
Amount unrestricted and permitted for investment payments	$ 251,700,000
Indentures \| Minimum
Long-term debt
Consolidated leverage ratio	1
Consolidated secured leverage ratio	1
Fixed charge coverage ratio	1
Indebtedness that may occur	$ 1
Indentures \| Maximum
Long-term debt
Consolidated leverage ratio	6
Consolidated secured leverage ratio	4
Fixed charge coverage ratio	2
2027 Notes Indentures \| Minimum
Long-term debt
Consolidated leverage ratio	1
Consolidated secured leverage ratio	4
Fixed charge coverage ratio	2
2027 Notes Indentures \| Maximum
Long-term debt
Consolidated leverage ratio	6
2032 Notes Indenture \| Minimum
Long-term debt
Consolidated leverage ratio	1
Fixed charge coverage ratio	2
2032 Notes Indenture \| Maximum
Long-term debt
Consolidated leverage ratio	6
Issuance Of Additional Class Notes [Member] \| Existing IPv4 Notes \| Minimum
Long-term debt
Consolidated leverage ratio			1
Issuance Of Additional Class Notes [Member] \| Existing IPv4 Notes \| Maximum
Long-term debt
Consolidated leverage ratio			6.1
Issuance Of Additional Class Notes [Member] \| IPv4 New Notes \| Minimum
Long-term debt
Consolidated leverage ratio		1
Issuance Of Additional Class Notes [Member] \| IPv4 New Notes \| Maximum
Long-term debt
Consolidated leverage ratio		7.25
Substitute New Ipv4 Addresses [Member] \| Existing IPv4 Notes \| Minimum
Long-term debt
Consolidated leverage ratio		1	1
Substitute New Ipv4 Addresses [Member] \| Existing IPv4 Notes \| Maximum
Long-term debt
Consolidated leverage ratio		7.25	6.1
Dispose Of Ipv Addresses Owned By Ipv Issuer Member Dispose Of Ipv Addresses Owned By Ipv Issuer [Member] \| Existing IPv4 Notes \| Minimum
Long-term debt
Consolidated leverage ratio			1
Dispose Of Ipv Addresses Owned By Ipv Issuer Member Dispose Of Ipv Addresses Owned By Ipv Issuer [Member] \| Existing IPv4 Notes \| Maximum
Long-term debt
Consolidated leverage ratio			7.1
Dispose Of Ipv Addresses Owned By Ipv Issuer Member Dispose Of Ipv Addresses Owned By Ipv Issuer [Member] \| IPv4 New Notes \| Minimum
Long-term debt
Consolidated leverage ratio		1
Dispose Of Ipv Addresses Owned By Ipv Issuer Member Dispose Of Ipv Addresses Owned By Ipv Issuer [Member] \| IPv4 New Notes \| Maximum
Long-term debt
Consolidated leverage ratio		7.25

COGENT COMMUNICATIONS HOLDINGS, INC. 10-Q