Postretirement Benefit Obligations - Narrative (Details) - USD ($) $ in Millions	1 Months Ended			3 Months Ended		4 Months Ended	5 Months Ended	9 Months Ended
	Oct. 31, 2024	Sep. 30, 2024	Mar. 31, 2024	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2023	May 17, 2023	Sep. 30, 2024
Defined Benefit Plan Disclosure [Line Items]
Excess assets from PA Mines UMWA Plan VEBA			$ 10
Gain on transfer of excess assets			$ 10
Contributions by employer		$ 38
Postretirement benefit prior service (credits) costs, net		$ 21		$ 21	$ 0	$ 0	$ 0	$ 21
Subsequent Event
Defined Benefit Plan Disclosure [Line Items]
Contributions by employer	$ 6