Net Earnings (Loss) Per Share (Calculation Of Net Earnings (Loss) Per Share) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Net Earnings (Loss) Per Share [Abstract]
Total basic net income (loss) attributable to Frontier common shareholders	$ 60	$ (186)
Total diluted net loss attributable to Frontier common shareholders	$ 60	$ (186)
Total weighted average shares and unvested restricted stock awards outstanding - basic (in shares)	104,786	105,060
Less: Weighted average unvested restricted stock awards	(230)	(697)
Total weighted average shares outstanding - basic	104,556	104,363
Basic net loss per share attributable to Frontier common shareholders	$ 0.57	$ (1.78)
Total weighted average shares outstanding - basic	104,556	104,363
Effect of dilutive stock units	340
Total weighted average shares outstanding - diluted	104,896	104,363
Diluted net loss per share attributable to Frontier common shareholders	$ 0.57	$ (1.78)