Related Party Transactions - Additional Information (Detail) - Wilmington Trust Company [Member] - USD ($)	12 Months Ended
Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Related Party Transaction [Line Items]
Indenture trustee fees	$ 0	$ 0	$ 0
Standard fees and commissions	$ 300,000	$ 300,000	$ 200,000

Those fees charged for services related to holding, advising, and managing customer investment assets. These account fees usually charged annually or semi-annually, on a per security basis, for the costs of services.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.3)
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

Name:	us-gaap_CustodyFees
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration