Long-Term Debt and Credit Facilities - Senior Secured Notes Issuance and Second Lien Tender Offers (Details) - USD ($) $ in Millions	Jun. 30, 2026	May 21, 2026	Mar. 31, 2026	Feb. 02, 2026	Jan. 09, 2026
Long-term Debt and Credit Facilities
Repurchased face amount	$ 14		$ 5	$ 4,765
Level 3 Financing, Inc.
Long-term Debt and Credit Facilities
Repurchased face amount		$ 766			$ 607
Level 3 Financing, Inc. \| Senior Notes \| 8.500% Senior Notes due 2036
Long-term Debt and Credit Facilities
Debt instrument, face amount					$ 650
Stated interest rate (as a percent)					8.50%
Level 3 Financing, Inc. \| Senior Notes \| 4.875% Second Lien Notes due 2029
Long-term Debt and Credit Facilities
Stated interest rate (as a percent)			4.875%		4.875%
Repurchased face amount			$ 1		$ 595
Level 3 Financing, Inc. \| Senior Notes \| 4.500% Second Lien Notes due 2030
Long-term Debt and Credit Facilities
Stated interest rate (as a percent)					4.50%
Repurchased face amount					$ 8
Level 3 Financing, Inc. \| Senior Notes \| 3.875% Second Lien Notes due 2030
Long-term Debt and Credit Facilities
Stated interest rate (as a percent)					3.875%
Repurchased face amount					$ 4