Investments in Real Estate-Related Securities - Additional Information (Detail)	3 Months Ended	4 Months Ended	6 Months Ended
Jun. 30, 2017 USD ($) Investment	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) Investment
Investment [Line Items]
Unrealized gain on investments in real estate related securities	$ 900,000		$ 1,600,000
Realized loss on CMBS investment repaid	177,000	$ 0	177,000
Securities sold	$ 0		$ 0
Commercial Mortgage Backed Securities Collateralized by Properties Owned by Blackstone-advised Investment Vehicles [Member]
Investment [Line Items]
Number of Investments \| Investment	5		5
Investments in securities, cost basis	$ 122,300,000		$ 122,300,000
Commercial Mortgage Backed Securities Collateralized by Loan Originated by Blackstone-advised Investment Vehicle [Member]
Investment [Line Items]
Number of Investments \| Investment	3		3
Investments in securities, cost basis	$ 63,500,000		$ 63,500,000
Interest income recorded	$ 1,200,000		$ 1,200,000
Commercial Mortgage Backed Securities [Member]
Investment [Line Items]
Number of Investments \| Investment	12		12
Investments in securities, cost basis	$ 289,914,000		$ 289,914,000
Maximum investment percentage in CMBS individual tranche	49.00%		49.00%

Maximum investment percentage in commercial mortgage backed securities individual tranche.

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Number of real estate related securities investment.

+ References

No definition available.

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 12
-Sentence Column A

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Amount before accretion (amortization) of purchase discount (premium) of interest income on nonoperating securities.

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The cash inflow associated with the sale of securities classified as trading securities and held for investment purposes. Excludes proceeds from trading securities purchased and held principally for the purpose of selling them in the near term (thus held for only a short period of time).

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Cost of investments in debt and equity securities and other forms of securities that provide ownership interests classified as trading.

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Amount of unrealized holding gain before loss recognized in the income statement for investments in trading securities.

+ References

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 210.5-03.7(c))
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 35
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82895926&loc=d3e24584-111560

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 9
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27357-111563

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