Net Loss Per Common Share (Calculation Of Net Loss Per Common Share) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Net Loss Per Common Share [Abstract]
Net loss attributable to Frontier common shareholders	$ (50)	$ 15	$ (181)	$ (186)	$ (162)	$ (345)	$ (5,317)	$ (87)	$ (402)	$ (5,911)	$ (750)
Total basic net loss attributable to Frontier common shareholders									(402)	(5,911)	(750)
Total diluted net loss attributable to Frontier common shareholders									$ (402)	$ (5,911)	$ (750)
Total weighted average shares and unvested restricted stock awards outstanding - basic (in shares)									104,944	105,356	91,523
Less: Weighted average unvested restricted stock awards									(477)	(1,291)	(1,840)
Total weighted average shares outstanding - basic									104,467	104,065	89,683
Basic net loss per share attributable to Frontier common shareholders									$ (3.85)	$ (56.80)	$ (8.37)
Total weighted average shares outstanding - diluted									104,467	104,065	89,683
Diluted net loss per share attributable to Frontier common shareholders									$ (3.85)	$ (56.80)	$ (8.37)