Retirement Plans (Net Periodic Benefit Cost) (Details) - USD ($) $ in Millions	1 Months Ended		3 Months Ended		4 Months Ended	5 Months Ended	9 Months Ended
	Aug. 31, 2021	May 31, 2021	Sep. 30, 2021	Sep. 30, 2020	Apr. 30, 2021	Sep. 30, 2021	Sep. 30, 2020
Successor [Member]
Defined Benefit Plan Disclosure [Line Items]
Amortization of prior service credit	$ 2	$ 55
Successor [Member] | Pension [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost			$ 20			$ 33
Interest cost on projected benefit obligation			26			44
Expected return on plan assets			(49)			(80)
Net periodic benefit cost			(3)			(3)
Total benefit cost						(3)
Successor [Member] | Postretirement [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost			4			7
Interest cost on projected benefit obligation			7			12
Amortization of prior service credit	$ 2	$ 55	(2)			(3)
Recognition of actuarial loss			54			67
Net periodic benefit cost			$ 63			83
Total benefit cost						$ 83
Predecessor [Member] | Pension [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost				$ 24	$ 32		$ 73
Interest cost on projected benefit obligation				26	31		84
Expected return on plan assets				(40)	(61)		(129)
Recognition of actuarial loss				29	24		76
Net periodic benefit cost				39	26		104
Pension settlement costs							159
Gain on disposal, net							(50)
Total benefit cost					26		213
Predecessor [Member] | Postretirement [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost				5	7		15
Interest cost on projected benefit obligation				8	9		24
Amortization of prior service credit				(8)	(10)		(24)
Recognition of actuarial loss				1	5		4
Net periodic benefit cost				$ 6	11		19
One-time gain on sale							(24)
Total benefit cost					$ 11		$ (5)