Postretirement Benefit Plans - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	3 Months Ended
	Jun. 30, 2024	Jun. 30, 2023
Components of net periodic benefit cost for postretirement benefit plans
Net periodic benefit cost	$ 1	$ 1
Pension Benefit Plans
Components of net periodic benefit cost for postretirement benefit plans
Service cost	5	5
Interest cost	19	19
Expected return on assets	(19)	(19)
Amortization — losses, net	0	(1)
Amortization — prior service credit, net	(1)	0
Net periodic benefit cost	4	4
Other Benefit Plans
Components of net periodic benefit cost for postretirement benefit plans
Service cost	0	0
Interest cost	1	1
Expected return on assets	0	0
Amortization — losses, net	0	0
Amortization — prior service credit, net	(1)	(1)
Net periodic benefit cost	$ 0	$ 0