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Long-Term Debt and Credit Facilities - 2025 Debt Transactions (Details) - USD ($) $ in Millions					3 Months Ended		9 Months Ended
	Sep. 14, 2025	Aug. 18, 2025	Jun. 30, 2025	Feb. 15, 2025	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024	Sep. 29, 2025	Sep. 08, 2025	Mar. 27, 2025	Dec. 31, 2024
Long-term Debt and Credit Facilities
Net loss (gain) on early retirement of debt					$ 395	$ 1	$ 666	$ (277)
Senior Notes
Long-term Debt and Credit Facilities
Aggregate principal amount of debt redeemed				$ 202
Level 3 Financing, Inc. \| Term Loan \| Term Loan B-3
Long-term Debt and Credit Facilities
Net loss (gain) on early retirement of debt					56		56
Level 3 Financing, Inc. \| Senior Notes \| First Lien Notes Due 2034
Long-term Debt and Credit Facilities
Net loss (gain) on early retirement of debt					344		344
Level 3 Financing, Inc. \| Senior Notes \| 11.000% Senior Secured Notes Due 2029
Long-term Debt and Credit Facilities
Stated interest rate		11.00%
Aggregate principal amount of debt redeemed		$ 1,400
Level 3 Financing, Inc. \| Senior Notes \| 10.750% First Lien Notes Due 2030
Long-term Debt and Credit Facilities
Stated interest rate	10.75%	10.75%
Aggregate principal amount of debt redeemed	$ 373	$ 305
Level 3 Financing, Inc. \| Senior Notes \| First Lien Notes Due 2033
Long-term Debt and Credit Facilities
Net loss (gain) on early retirement of debt							236
Level 3 Financing, Inc. \| Senior Notes \| 10.500% Senior Secured Notes Due 2030
Long-term Debt and Credit Facilities
Stated interest rate			10.50%
Aggregate principal amount of debt redeemed			$ 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%
Aggregate principal amount of debt redeemed			$ 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%
Aggregate principal amount of debt redeemed			$ 167
Level 3 Financing, Inc. \| Term Loan \| Term Loan B-4
Long-term Debt and Credit Facilities
Long-term debt, gross					$ 2,400		$ 2,400		$ 2,400			$ 0
Level 3 Financing, Inc. \| Term Loan \| Term Loan B-3
Long-term Debt and Credit Facilities
Long-term debt, gross											$ 2,400
Level 3 Financing, Inc. \| Senior Notes \| First Lien Notes Due 2034
Long-term Debt and Credit Facilities
Face amount		$ 2,000								$ 425
Stated interest rate		7.00%								7.00%
Level 3 Financing, Inc. \| Senior Notes \| First Lien Notes Due 2033
Long-term Debt and Credit Facilities
Face amount			$ 2,000
Stated interest rate			6.875%