Related Party Transactions - Additional Information (Details) - USD ($) $ in Millions	Dec. 31, 2017	Mar. 31, 2017
TMS [Member]
Related Party Transaction [Line Items]
Subvention support receivable	$ 248	$ 165

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