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