Long-Term Debt and Credit Facilities - 2025 Debt Transactions (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024	Dec. 31, 2024
Long-term Debt and Credit Facilities
Net loss (gain) on early retirement of debt	$ 236	$ (3)	$ 271	$ (278)
Level 3 Financing, Inc. | Senior Notes | First Lien Notes
Long-term Debt and Credit Facilities
Face amount	$ 2,000		$ 2,000
Stated interest rate	6.875%		6.875%
Long-term debt, gross	$ 4,087		$ 4,087		$ 3,846
Level 3 Financing, Inc. | Senior Notes | 10.500% Senior Secured Notes due 2030
Long-term Debt and Credit Facilities
Stated interest rate	10.50%		10.50%
Long-term debt, gross	$ 925		$ 925
Level 3 Financing, Inc. | Senior Notes | 10.500% First Lien Notes Due 2029
Long-term Debt and Credit Facilities
Stated interest rate	10.50%		10.50%
Long-term debt, gross	$ 668		$ 668
Level 3 Financing, Inc. | Senior Notes | 11.000% First Lien Notes Due 2029
Long-term Debt and Credit Facilities
Stated interest rate	11.00%		11.00%
Long-term debt, gross	$ 167		$ 167
Level 3 Financing, Inc. | Senior Notes | First Lien Notes Refinancing
Long-term Debt and Credit Facilities
Net loss (gain) on early retirement of debt	$ 236		$ 236