Pension and Other Benefit Plans - Schedule of Components of Net Periodic Pension Expense (Details) $ in Millions	12 Months Ended
	Mar. 31, 2019 USD ($)
Defined Benefit Pension Plan
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	$ 0
Interest cost	16
Expected return on assets	(24)
Recognition of net actuarial loss	35
Net periodic pension expense (income)	27
Retiree Medical Plan
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	0
Interest cost	0
Expected return on assets	(1)
Recognition of net actuarial loss	0
Net periodic pension expense (income)	$ (1)