Comprehensive Income (Loss) (Reclassification Out Of AOCI) (Details) - USD ($) $ 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] | Pension [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications, net of tax
Successor [Member] | OPEB [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications, net of tax				2
Successor [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member] | OPEB [Member] | Amortization Of Defined Benefit Cost Items [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications, pretax	$ 2			3
Tax impact	(1)			(1)
Reclassifications, net of tax	1			2
Successor [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member] | OPEB [Member] | Prior-Service Credits (Costs) [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications, pretax	$ 2			$ 3
Predecessor [Member] | Pension [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications, net of tax			$ (19)		$ (233)
Predecessor [Member] | OPEB [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications, net of tax			4		10
Predecessor [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member] | Pension [Member] | Amortization Of Defined Benefit Cost Items [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications, pretax		$ (29)	(24)		(296)
Tax impact		4	5		63
Reclassifications, net of tax		(25)	(19)		(233)
Predecessor [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member] | Pension [Member] | Actuarial Losses [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications, pretax		(29)	(24)		(76)
Predecessor [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member] | Pension [Member] | Loss On Disposal [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications, pretax					(61)
Predecessor [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member] | Pension [Member] | Pension Settlement Cost [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications, pretax					(159)
Predecessor [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member] | OPEB [Member] | Amortization Of Defined Benefit Cost Items [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications, pretax		7	5		13
Tax impact		(2)	(1)		(3)
Reclassifications, net of tax		5	4		10
Predecessor [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member] | OPEB [Member] | Prior-Service Credits (Costs) [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications, pretax		8	10		24
Predecessor [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member] | OPEB [Member] | Actuarial Losses [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications, pretax		$ (1)	$ (5)		(4)
Predecessor [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member] | OPEB [Member] | Loss On Disposal [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications, pretax					$ (7)