Net Income Per Common Share (Reconciliation Of Net Loss Per Share) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	4 Months Ended	8 Months Ended	12 Months Ended
	Apr. 30, 2021	Dec. 31, 2021	Dec. 31, 2023	Dec. 31, 2022
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Net income attributable to Frontier common shareholders	$ 4,541	$ 414	$ 29	$ 441
Total basic net income attributable to Frontier common shareholders	4,541	414	29	441
Total diluted net income attributable to Frontier common shareholders	$ 4,541	$ 414	$ 29	$ 441
Total weighted average shares and unvested restricted stock awards outstanding - basic (in shares)	104,799	244,405	245,517	244,781
Less: Weighted average unvested restricted stock awards	(215)
Total weighted average shares outstanding - basic	104,584	244,405	245,517	244,781
Basic net earnings per share attributable to Frontier common shareholders	$ 43.42	$ 1.69	$ 0.12	$ 1.80
Total weighted average shares outstanding - basic	104,584	244,405	245,517	244,781
Total weighted average shares outstanding - diluted	104,924	245,885	248,459	245,280
Diluted net earnings per share attributable to Frontier common shareholders	$ 43.28	$ 1.68	$ 0.12	$ 1.80
Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Effect of dilutive awards (in shares)	340	1,480	2,330
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Effect of dilutive awards (in shares)			612	499