Investments in Real Estate-Related Securities - Summary of Investments in CMBS (Parenthetical) (Detail) $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2018 USD ($) Position	Dec. 31, 2017 USD ($)
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Fair Value	$ 2,029,334	$ 915,742
One-Month LIBOR [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Investment interest rate	2.30%	1.60%
Commercial Mortgage Backed Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Fair Value	$ 2,029,334	$ 915,742
Commercial Mortgage Backed Securities [Member] \| Multifamily [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Fair Value	$ 7,447	$ 11,737
Number of interest-only positions \| Position	2
Notional amount of interest-only positions	$ 1,200,000
Commercial Mortgage Backed Securities [Member] \| Corporate Bond Securities [Member] \| Hospitality, Office, Multifamily, Retail [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Number of corporate bond position \| Position	1
Fair Value	$ 54,000
Commercial Mortgage Backed Securities [Member] \| One-Month LIBOR [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Investment interest rate	2.30%	1.60%
Weighted average coupon position \| Position	1
Weighted Average Coupon	2.20%

Number of corporate bond position.

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- Definition

Number of interest-only positions.

+ References

No definition available.

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- Definition

Number of weighted average coupon position.

+ References

No definition available.

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- Definition

Real estate related securities interest only notional amount.

+ References

No definition available.

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- Definition

Weighted average coupon basis spread on variable rate.

+ References

No definition available.

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- Definition

The interest rate for investments that have an interest rate. For fixed rate investments, this indicates the fixed interest rate. If the investment has a variable interest rate, the rate stated here may be the rate that is currently relevant. In this case the "Investment, Interest Rate is Current Rate for Variable Rate Investment Flag", "Investment, Interest Rate is Discount Rate at Purchase Flag", "Investment, Interest Rate Reflects Current Yield Flag" should be used. If one of these flags are not appropriate to explain the interest rate that is indicated, then a new flag should be added or an explanation should be included in "Investment, Additional Information".

+ 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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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in net income (trading) and investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.4)
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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