Net (Loss) Earnings Per Share (Reconciliation Of Net (Loss) Income Per Share) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total basic net income (loss) attributable to Frontier common shareholders	$ (64)	$ 1
Effect of loss related to dilutive stock units
Total diluted net income (loss) attributable to Frontier common shareholders	$ (64)	$ 1
Total weighted average shares and unvested restricted stock awards outstanding - basic (in shares)	249,830	246,301
Less: Weighted average unvested restricted stock awards
Total weighted average shares outstanding - basic	249,830	246,301
Basic net earnings (loss) per share attributable to Frontier common shareholders	$ (0.26)	$ 0.00
Total weighted average shares outstanding - basic	249,830	246,301
Total weighted average shares outstanding - diluted	249,830	247,040
Diluted net earnings (loss) per share attributable to Frontier common shareholders	$ (0.26)	$ 0.00
Performance Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Effect of dilutive awards (in shares)		72
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Effect of dilutive awards (in shares)		667