Comprehensive Income (Loss) (Accumulated Other Comprehensive Income (Loss), Net Of Tax) (Details) - USD ($) $ in Millions	1 Months Ended	2 Months Ended	3 Months Ended					4 Months Ended	5 Months Ended	9 Months Ended
	Apr. 30, 2021	Jun. 30, 2021	Sep. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Apr. 30, 2021	Sep. 30, 2021	Sep. 30, 2020	Jan. 01, 2021	Jan. 01, 2020
Successor [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning		$ 4,108	$ 4,248						$ 4,108
Net current-period other comprehensive income (loss)		41	3						44
Balance at ending	$ 4,108	4,248	4,385					$ 4,108	4,385
Successor [Member] | Pension [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning
Other comprehensive income (loss) before reclassifications
Amounts reclassified from accumulated other comprehensive income (loss)
Net current-period other comprehensive income (loss)
Balance at ending
Successor [Member] | OPEB [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning
Other comprehensive income (loss) before reclassifications									46
Amounts reclassified from accumulated other comprehensive income (loss)									(2)
Net current-period other comprehensive income (loss)									44
Balance at ending			44						44
Successor [Member] | Pension And OPEB [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss) before reclassifications									46
Amounts reclassified from accumulated other comprehensive income (loss)									(2)
Net current-period other comprehensive income (loss)									44
Balance at ending			44						44
Accumulated other comprehensive income (loss), net of tax			12						12
Successor [Member] | Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning			41
Net current-period other comprehensive income (loss)		41	3
Balance at ending		41	$ 44						44
Predecessor [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning	(4,830)	4,108		$ (4,900)	$ (5,097)	$ (4,493)	$ (4,394)	(4,900)	4,108	$ (4,394)
Net current-period other comprehensive income (loss)	348			11	155	(423)	86	359		(182)
Balance at ending	4,108			(4,830)	(4,926)	(5,097)	(4,493)	4,108		(4,926)
Predecessor [Member] | Pension [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning				(699)			(684)	(699)		(684)
Other comprehensive income (loss) before reclassifications								270		(389)
Amounts reclassified from accumulated other comprehensive income (loss)								19		233
Net current-period other comprehensive income (loss)								289		(156)
Cancellation of Predecessor equity								410
Balance at ending					(840)					(840)
Predecessor [Member] | OPEB [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning				(56)			34	(56)		34
Other comprehensive income (loss) before reclassifications								74		(16)
Amounts reclassified from accumulated other comprehensive income (loss)								(4)		(10)
Net current-period other comprehensive income (loss)								70		(26)
Cancellation of Predecessor equity								(14)
Balance at ending					8					8
Predecessor [Member] | Pension And OPEB [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning				(755)			(650)	(755)		(650)
Other comprehensive income (loss) before reclassifications								344		(405)
Amounts reclassified from accumulated other comprehensive income (loss)								15		223
Net current-period other comprehensive income (loss)								359		(182)
Cancellation of Predecessor equity								396
Balance at ending					(832)					(832)
Accumulated other comprehensive income (loss), net of tax					255					255	$ 234	$ 204
Predecessor [Member] | Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning	(744)			(755)	(987)	(564)	(650)	(755)		(650)
Net current-period other comprehensive income (loss)	348			11	155	(423)	86
Balance at ending				$ (744)	$ (832)	$ (987)	$ (564)			$ (832)