Commitments and Contingencies - Additional Information (Detail)	Sep. 30, 2018 Investment
Commitments And Contingencies Disclosure [Abstract]
Number of real estate investments subject to ground leases	3

Number of separate real estate development properties located on land subject to ground leases.

No definition available.

Name:	us-gaap_NumberOfPropertiesSubjectToGroundLeases
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant