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