Background - Additional Information (Detail) - Trust	12 Months Ended
Dec. 31, 2016	Oct. 24, 2014	Aug. 03, 2006
Related Party Transactions [Abstract]
Number of series of trusts		2	2
Purchase agreement transaction date	Feb. 23, 2015

Date when the business acquisition agreement was executed, in CCYY-MM-DD format.

No definition available.

Name:	us-gaap_BusinessAcquisitionDateOfAcquisitionAgreement1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration