Background - Additional Information (Detail) - Jun. 30, 2015 - Trust	Total
Related Party Transactions [Abstract]
Number of series of trust	7
Purchase agreement transaction date	Feb. 23, 2015

Date of acquisition or disposal, in CCYY-MM-DD format.

No definition available.

Name:	us-gaap_SignificantAcquisitionsAndDisposalsDateOfTransactionForAcquisitionsOrDisposals
Namespace Prefix:	us-gaap_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration