Pension and Other Benefit Plans - Reconciliation of Projected Benefit Obligation (Details) - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	$ 247
Fair value of plan assets at end of year	155	$ 247
Defined Benefit Pension Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Projected benefit obligation at beginning of year	491	0
Benefit obligation assumed as a result of the Mergers	0	482
Service cost	0	0
Interest cost	18	16
Actuarial loss (gain)	35	13
Plan participant contributions	0	0
Benefits paid	(21)	(20)
Projected benefit obligation at end of year	523	491
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	388	0
Assets assumed as a result of the Mergers	0	405
Actual return on plan assets	(8)	3
Plan participant contributions	0	0
Expenses paid	(2)	0
Benefits paid	(21)	(20)
Fair value of plan assets at end of year	357	388
Funded status at end of year	(166)	(103)
Retiree Medical Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Projected benefit obligation at beginning of year	7	0
Benefit obligation assumed as a result of the Mergers	0	7
Service cost	0	0
Interest cost	0	0
Actuarial loss (gain)	0	1
Plan participant contributions	0	1
Benefits paid	0	(2)
Projected benefit obligation at end of year	7	7
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	13	0
Assets assumed as a result of the Mergers	0	14
Actual return on plan assets	(1)	0
Plan participant contributions	0	1
Expenses paid	0	0
Benefits paid	0	(2)
Fair value of plan assets at end of year	12	13
Funded status at end of year	$ 5	$ 6