Pension and Other Benefit Plans - Reconciliation of Assets Valued Using Significant Unobservable Inputs (Details) - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]
Fair value of plan assets at beginning of year	$ 247
Fair value of plan assets at end of year	155	$ 247
Level 3
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]
Fair value of plan assets at beginning of year	78	0
Assets assumed as a result of the Mergers		77
Actual return on plan assets held at the reporting date	1	2
Purchases, sales, and settlements	(25)	(1)
Fair value of plan assets at end of year	54	78
Real Estate Funds
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]
Fair value of plan assets at beginning of year	53
Fair value of plan assets at end of year	46	53
Real Estate Funds | Level 3
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]
Fair value of plan assets at beginning of year	53	0
Assets assumed as a result of the Mergers		49
Actual return on plan assets held at the reporting date	2	5
Purchases, sales, and settlements	(9)	(1)
Fair value of plan assets at end of year	46	53
Hedge Funds
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]
Fair value of plan assets at beginning of year	25
Fair value of plan assets at end of year	8	25
Hedge Funds | Level 3
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]
Fair value of plan assets at beginning of year	25	0
Assets assumed as a result of the Mergers		28
Actual return on plan assets held at the reporting date	(1)	(3)
Purchases, sales, and settlements	(16)	0
Fair value of plan assets at end of year	$ 8	$ 25