Postretirement Benefit Plans - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2023	Dec. 31, 2022
Components of net periodic benefit cost for postretirement benefit plans
Net periodic benefit cost	$ 1	$ 0	$ 4	$ 1
Pension Benefit Plans
Components of net periodic benefit cost for postretirement benefit plans
Service cost	6	7	17	20
Interest cost	19	16	57	47
Expected return on assets	(20)	(18)	(59)	(54)
Amortization — losses, net	0	2	(1)	6
Amortization — prior service credit, net	0	(1)	(1)	(1)
Net periodic benefit cost	5	6	13	18
Other Benefit Plans
Components of net periodic benefit cost for postretirement benefit plans
Service cost	1	1	2	3
Interest cost	2	2	5	4
Expected return on assets	0	0	0	0
Amortization — losses, net	(1)	(1)	(2)	(1)
Amortization — prior service credit, net	(1)	0	(3)	(2)
Net periodic benefit cost	$ 1	$ 2	$ 2	$ 4