Retirement Plans (Narrative) (Details) - USD ($) $ in Millions	1 Months Ended		3 Months Ended		4 Months Ended	5 Months Ended	9 Months Ended		12 Months Ended
	Aug. 31, 2021	May 31, 2021	Sep. 30, 2021	Sep. 30, 2020	Apr. 30, 2021	Sep. 30, 2021	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2021	Dec. 31, 2020	Jul. 31, 2021	Mar. 31, 2021
Successor [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Capitalization of pension and OPEB expense						$ 10
Prior service credit	$ 2	$ 55
Predecessor [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Capitalization of pension and OPEB expense			$ 6	$ 5	$ 7			$ 18
Increase in plan assets										$ 79
Plan assets										$ 2,507
Pension [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Amortization period							15 years			7 years
Pension [Member] | Successor [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Investment returns						68
Plan assets			2,598			2,598	$ 2,598
Benefit payments						51
Plan expenses						5
Pension [Member] | Predecessor [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate (in hundredths)					3.10%					2.60%
Rate of return on plan assets (in hundredths)					7.50%
Pension obligation decrease					$ 328
Defined Benefit Plan, Settlements Benefit obligation threshold								211
Pension settlement costs								159
Contributions to plans					32					$ 64
Funding waiver amount and will amortized over next five years										127
Increase in plan assets					12
Investment returns					78
Plan assets					2,586
Benefit payments					25
Plan expenses					6
Settlements								465
Pension [Member] | Scenario, Forecast [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Contributions to plans									$ 42
Postretirement [Member] | Successor [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate (in hundredths)	2.80%	3.20%									3.20%
Benefit obligation					$ 101
Prior service credit	$ 2	$ 55	$ (2)			$ (3)
Actuarial loss	$ 54	$ 13
Postretirement [Member] | Predecessor [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate (in hundredths)					3.30%							2.60%
Benefit obligation					$ 941					$ 1,042
Prior service credit				$ (8)	$ (10)			$ (24)