Long-Term Debt and Credit Facilities - Debt New Issuances (Details) $ in Millions	Mar. 31, 2024 USD ($)
Long-term Debt and Credit Facilities
Aggregate principal amount	$ 11,982
Term Loan A | Term Loan
Long-term Debt and Credit Facilities
Aggregate principal amount	372
Term Loan B-1 | Term Loan
Long-term Debt and Credit Facilities
Aggregate principal amount	1,625
Term Loan B-2 | Term Loan
Long-term Debt and Credit Facilities
Aggregate principal amount	$ 1,625
4.125% Senior Secured Notes Due 2029-2030 | Senior Notes
Long-term Debt and Credit Facilities
Stated interest rate	4.125%
Aggregate principal amount	$ 812
Term Loan B | Term Loan
Long-term Debt and Credit Facilities
Installment payments	13
Level 3 Financing, Inc. | Term Loan B-1 | Term Loan
Long-term Debt and Credit Facilities
Aggregate principal amount	1,199
Level 3 Financing, Inc. | Term Loan B-2 | Term Loan
Long-term Debt and Credit Facilities
Aggregate principal amount	$ 1,199
Level 3 Financing, Inc. | 10.500% First Lien Notes Due 2029 | Senior Notes
Long-term Debt and Credit Facilities
Stated interest rate	10.50%
Aggregate principal amount	$ 668
Level 3 Financing, Inc. | 10.750% First Lien Notes Due 2030 | Senior Notes
Long-term Debt and Credit Facilities
Stated interest rate	10.75%
Aggregate principal amount	$ 678
Level 3 Financing, Inc. | 11.000% First Lien Notes Due 2029 | Senior Notes
Long-term Debt and Credit Facilities
Stated interest rate	11.00%
Aggregate principal amount	$ 1,375
Level 3 Financing, Inc. | 11.000% First Lien Notes Due 2029 | Senior Notes
Long-term Debt and Credit Facilities
Stated interest rate	11.00%
Aggregate principal amount	$ 200
Level 3 Financing, Inc. | 4.875% Second Lien Notes Due 2029 | Senior Notes
Long-term Debt and Credit Facilities
Stated interest rate	4.875%
Aggregate principal amount	$ 606
Level 3 Financing, Inc. | 4.500% Second Lien Notes Due 2030 | Senior Notes
Long-term Debt and Credit Facilities
Stated interest rate	4.50%
Aggregate principal amount	$ 712
Level 3 Financing, Inc. | 3.875% Second Lien Notes Due 2030 | Senior Notes
Long-term Debt and Credit Facilities
Stated interest rate	3.875%
Aggregate principal amount	$ 458
Level 3 Financing, Inc. | 4.000% Second Lien Notes Due 2031 | Senior Notes
Long-term Debt and Credit Facilities
Stated interest rate	4.00%
Aggregate principal amount	$ 453