Long-Term Debt (Long-Term Debt) (Details) - USD ($) $ in Millions	4 Months Ended	8 Months Ended	12 Months Ended
	Apr. 30, 2021	Dec. 31, 2021	Dec. 31, 2023	Dec. 31, 2022
Debt Instrument [Line Items]
Principal debt outstanding, beginning			$ 8,963
Principal Payments and Retirements			(68)
New Borrowings			2,336
Principal debt outstanding, ending			11,231	$ 8,963
Less: Debt issuance costs			(71)	(28)
Less: Debt premium (discount)			(64)
Less: Current portion			(15)	(15)
Plus: Unamortized fair value adjustments			165	190
Long-term debt			$ 11,246	9,110
Long-term debt, fair value adjustment	$ (236)
Interest Rate			7.103%
Net proceeds from long-term debt borrowings	$ 225	$ 1,000	$ 2,278	1,200
Secured Debt Issued By Frontier [Member]
Debt Instrument [Line Items]
Principal debt outstanding, beginning			8,113
Principal Payments and Retirements			(15)
New Borrowings			750
Principal debt outstanding, ending			$ 8,848	8,113
Interest Rate			7.001%
Secured Debt Issued By Subsidiaries [Member]
Debt Instrument [Line Items]
Principal debt outstanding, beginning			$ 100
Principal Payments and Retirements			(53)
New Borrowings			1,586
Principal debt outstanding, ending			$ 1,633	100
Interest Rate			7.751%
Unsecured Debt Issued By Subsidiaries [Member]
Debt Instrument [Line Items]
Principal debt outstanding, beginning			$ 750
Principal debt outstanding, ending			$ 750	$ 750
Interest Rate			6.899%