Long-Term Debt (Long-Term Debt) (Details) - USD ($) $ in Millions	6 Months Ended
Jun. 30, 2025	Dec. 31, 2024
Debt Instrument [Line Items]
Principal debt outstanding, beginning	$ 11,569
Principal Payments and Retirements	(3)
New Borrowings	330
Principal debt outstanding, ending	11,896
Less: Debt issuance costs	(96)	$ (89)
Less: Current portion	(10)	(10)
Less: Debt premium / (discount)	(38)	(44)
Plus: Unamortized fair value adjustments	108	125
Long-term debt	11,860	$ 11,551
Long-term debt, fair value adjustment	236
Secured Debt Issued By Frontier [Member]
Debt Instrument [Line Items]
Principal debt outstanding, beginning	8,436
Principal Payments and Retirements	(3)
Principal debt outstanding, ending	8,433
Secured Debt Issued By Subsidiaries [Member]
Debt Instrument [Line Items]
Principal debt outstanding, beginning	2,383
New Borrowings	330
Principal debt outstanding, ending	2,713
Unsecured Debt Issued By Subsidiaries [Member]
Debt Instrument [Line Items]
Principal debt outstanding, beginning	750
Principal debt outstanding, ending	$ 750

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Long Term Debt Fair Value Adjustment

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