Long-Term Debt and Credit Facilities - Debt New Issuances (Details) $ in Millions	6 Months Ended
	Sep. 30, 2024 USD ($)
Long-term Debt and Credit Facilities
Aggregate principal amount	$ 11,938
Senior Notes
Long-term Debt and Credit Facilities
Aggregate principal amount	848
Term Loan
Long-term Debt and Credit Facilities
Installment payments	$ 53
5.125% senior notes due 2026 | Senior Notes
Long-term Debt and Credit Facilities
Stated interest rate	5.125%
Aggregate principal amount	$ 137
4.000% senior secured notes due 2027 (unsecured) | Senior Notes
Long-term Debt and Credit Facilities
Stated interest rate	4.00%
Aggregate principal amount	$ 188
6.875% debentures, series G, due 2028 | Senior Notes
Long-term Debt and Credit Facilities
Stated interest rate	6.875%
Aggregate principal amount	$ 80
4.500% senior notes due 2029 | Senior Notes
Long-term Debt and Credit Facilities
Stated interest rate	4.50%
Aggregate principal amount	$ 86
Term Loan A | Term Loan
Long-term Debt and Credit Facilities
Aggregate principal amount	362
Term Loan B-1 | Term Loan
Long-term Debt and Credit Facilities
Aggregate principal amount	1,610
Term Loan B-2 | Term Loan
Long-term Debt and Credit Facilities
Aggregate principal amount	$ 1,610
4.125% Superpriority Notes due 2029-2030 | Senior Notes
Long-term Debt and Credit Facilities
Stated interest rate	4.125%
Aggregate principal amount	$ 808
Level 3 Financing, Inc. | 3.400% senior secured notes due 2027 (unsecured) | Senior Notes
Long-term Debt and Credit Facilities
Stated interest rate	3.40%
Aggregate principal amount	$ 77
Level 3 Financing, Inc. | 4.625% senior notes due 2027 | Senior Notes
Long-term Debt and Credit Facilities
Stated interest rate	4.625%
Aggregate principal amount	$ 280
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 2029 | 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	$ 1,575
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. | 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