Related Party Transactions - Additional Information (Details) - USD ($) $ in Millions	Sep. 30, 2018	Mar. 31, 2018
TMNA [Member]
Related Party Transaction [Line Items]
Subvention support receivable	$ 197	$ 279

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

No definition available.

Name:	us-gaap_RelatedPartyTransactionLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration