Net Earnings (Loss) Per Share (Reconciliation Of Net Earnings (Loss) Per Share) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended		4 Months Ended	5 Months Ended	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Apr. 30, 2021	Sep. 30, 2021	Sep. 30, 2020
Successor [Member]
Total basic net income (loss) attributable to Frontier common shareholders	$ 126			$ 225
Total diluted net income (loss) attributable to Frontier common shareholders	$ 126			$ 225
Total weighted average shares and unvested restricted stock awards outstanding - basic (in shares)	244,403			244,402
Total weighted average shares outstanding - basic	244,403			244,402
Basic net earnings (loss) per share attributable to Frontier common shareholders	$ 0.52			$ 0.92
Total weighted average shares outstanding - basic	244,403			244,402
Effect of dilutive stock units (in shares)	1,264			1,198
Total weighted average shares outstanding - diluted	245,667			245,600
Diluted net earnings (loss) per share attributable to Frontier common shareholders	$ 0.51			$ 0.92
Predecessor [Member]
Total basic net income (loss) attributable to Frontier common shareholders		$ 15	$ 4,541		$ (352)
Total diluted net income (loss) attributable to Frontier common shareholders		$ 15	$ 4,541		$ (352)
Total weighted average shares and unvested restricted stock awards outstanding - basic (in shares)		104,919	104,799		104,989
Less: Weighted average unvested restricted stock awards		(393)	(215)		(529)
Total weighted average shares outstanding - basic		104,526	104,584		104,460
Basic net earnings (loss) per share attributable to Frontier common shareholders		$ 0.14	$ 43.42		$ (3.37)
Total weighted average shares outstanding - basic		104,526	104,584		104,460
Effect of dilutive stock units (in shares)		340	340
Total weighted average shares outstanding - diluted		104,866	104,924		104,460
Diluted net earnings (loss) per share attributable to Frontier common shareholders		$ 0.14	$ 43.28		$ (3.37)