Fair Value and Future Funding Commitments Related to These Investments (Parenthetical) (Detail)	9 Months Ended
Sep. 30, 2018 USD ($)
Fair Value Disclosures [Abstract]
Fair value written down	$ 0

Amount of write-down or reserve recognized in the income statement on real estate investments for entities with a substantial portion of business acquiring and holding investment real estate.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 970
-SubTopic 360
-Section S99
-Paragraph 1
-Subparagraph (SX 210.12-28(Footnote 5))
-URI http://asc.fasb.org/extlink&oid=6590653&loc=d3e638233-123024

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