Schedule IV - Mortgage Loans on Real Estate (Detail) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Mortgage Loans on Real Estate [Line Items]
Interest Payment Rates	1.90%
Prior Liens	$ 985,382
Face Amount of Loans	10,108,226	$ 8,727,218
Carrying Amount of Loans	$ 10,056,732
Senior Mortgage Loans in Excess of 3% of the Carrying Amount of Total Loans [Member] \| Borrower A [Member] \| Office [Member]
Mortgage Loans on Real Estate [Line Items]
Location	Office / Southeast
Debt instrument variable rate basis	LIBOR
Interest Payment Rates	3.40%
Maximum Maturity Date	2023
Periodic Payment Terms	I/O
Face Amount of Loans	$ 656,406
Carrying Amount of Loans	652,880
Senior Mortgage Loans Less Than 3% of the Carrying Amount of Total Loans [Member]
Mortgage Loans on Real Estate [Line Items]
Face Amount of Loans	9,237,199
Carrying Amount of Loans	$ 9,190,208
Senior Mortgage Loans Less Than 3% of the Carrying Amount of Total Loans [Member] \| Hotel [Member]
Mortgage Loans on Real Estate [Line Items]
Location	Hotel / Diversified
Debt instrument variable rate basis	LIBOR
Periodic Payment Terms	I/O&P/I
Face Amount of Loans	$ 1,843,143
Carrying Amount of Loans	$ 1,837,222
Senior Mortgage Loans Less Than 3% of the Carrying Amount of Total Loans [Member] \| Hotel [Member] \| Minimum [Member]
Mortgage Loans on Real Estate [Line Items]
Interest Payment Rates	3.25%
Maximum Maturity Date	2018
Senior Mortgage Loans Less Than 3% of the Carrying Amount of Total Loans [Member] \| Hotel [Member] \| Maximum [Member]
Mortgage Loans on Real Estate [Line Items]
Interest Payment Rates	5.16%
Maximum Maturity Date	2022
Senior Mortgage Loans Less Than 3% of the Carrying Amount of Total Loans [Member] \| Office [Member]
Mortgage Loans on Real Estate [Line Items]
Location	Office / Diversified
Debt instrument variable rate basis	LIBOR
Periodic Payment Terms	I/O&P/I
Face Amount of Loans	$ 5,357,888
Carrying Amount of Loans	$ 5,327,333
Senior Mortgage Loans Less Than 3% of the Carrying Amount of Total Loans [Member] \| Office [Member] \| Minimum [Member]
Mortgage Loans on Real Estate [Line Items]
Interest Payment Rates	3.00%
Fixed Interest Payment Rates	2.30%
Maximum Maturity Date	2018
Senior Mortgage Loans Less Than 3% of the Carrying Amount of Total Loans [Member] \| Office [Member] \| Maximum [Member]
Mortgage Loans on Real Estate [Line Items]
Interest Payment Rates	5.75%
Fixed Interest Payment Rates	5.29%
Maximum Maturity Date	2023
Senior Mortgage Loans Less Than 3% of the Carrying Amount of Total Loans [Member] \| Retail [Member]
Mortgage Loans on Real Estate [Line Items]
Location	Retail / Diversified
Debt instrument variable rate basis	LIBOR
Fixed Interest Payment Rates	5.14%
Periodic Payment Terms	I/O&P/I
Face Amount of Loans	$ 319,223
Carrying Amount of Loans	$ 318,789
Senior Mortgage Loans Less Than 3% of the Carrying Amount of Total Loans [Member] \| Retail [Member] \| Minimum [Member]
Mortgage Loans on Real Estate [Line Items]
Interest Payment Rates	3.10%
Maximum Maturity Date	2019
Senior Mortgage Loans Less Than 3% of the Carrying Amount of Total Loans [Member] \| Retail [Member] \| Maximum [Member]
Mortgage Loans on Real Estate [Line Items]
Interest Payment Rates	4.25%
Maximum Maturity Date	2021
Senior Mortgage Loans Less Than 3% of the Carrying Amount of Total Loans [Member] \| Multifamily [Member]
Mortgage Loans on Real Estate [Line Items]
Location	Multifamily / Diversified
Debt instrument variable rate basis	LIBOR
Periodic Payment Terms	I/O&P/I
Face Amount of Loans	$ 1,122,532
Carrying Amount of Loans	$ 1,114,353
Senior Mortgage Loans Less Than 3% of the Carrying Amount of Total Loans [Member] \| Multifamily [Member] \| Minimum [Member]
Mortgage Loans on Real Estate [Line Items]
Interest Payment Rates	2.80%
Maximum Maturity Date	2018
Senior Mortgage Loans Less Than 3% of the Carrying Amount of Total Loans [Member] \| Multifamily [Member] \| Maximum [Member]
Mortgage Loans on Real Estate [Line Items]
Interest Payment Rates	5.25%
Maximum Maturity Date	2023
Senior Mortgage Loans Less Than 3% of the Carrying Amount of Total Loans [Member] \| Condo / Northeast [Member]
Mortgage Loans on Real Estate [Line Items]
Location	Condo / Northeast
Debt instrument variable rate basis	LIBOR
Interest Payment Rates	4.15%
Maximum Maturity Date	2021
Periodic Payment Terms	I/O
Face Amount of Loans	$ 66,451
Carrying Amount of Loans	$ 65,775
Senior Mortgage Loans Less Than 3% of the Carrying Amount of Total Loans [Member] \| Other [Member]
Mortgage Loans on Real Estate [Line Items]
Location	Other / Diversified
Debt instrument variable rate basis	LIBOR
Periodic Payment Terms	I/O&P/I
Face Amount of Loans	$ 527,962
Carrying Amount of Loans	$ 526,736
Senior Mortgage Loans Less Than 3% of the Carrying Amount of Total Loans [Member] \| Other [Member] \| Minimum [Member]
Mortgage Loans on Real Estate [Line Items]
Interest Payment Rates	3.10%
Fixed Interest Payment Rates	4.63%
Maximum Maturity Date	2018
Senior Mortgage Loans Less Than 3% of the Carrying Amount of Total Loans [Member] \| Other [Member] \| Maximum [Member]
Mortgage Loans on Real Estate [Line Items]
Interest Payment Rates	5.00%
Fixed Interest Payment Rates	5.20%
Maximum Maturity Date	2022
Senior Mortgage Loans [Member]
Mortgage Loans on Real Estate [Line Items]
Face Amount of Loans	$ 9,893,605
Carrying Amount of Loans	9,843,088
Subordinate Loans [Member]
Mortgage Loans on Real Estate [Line Items]
Prior Liens	985,382
Face Amount of Loans	214,621
Carrying Amount of Loans	$ 213,644
Subordinate Loans [Member] \| Various / Diversified [Member]
Mortgage Loans on Real Estate [Line Items]
Debt instrument variable rate basis	LIBOR
Fixed Interest Payment Rates	12.22%
Periodic Payment Terms	I/O & P/I
Prior Liens	$ 985,382
Face Amount of Loans	214,621
Carrying Amount of Loans	$ 213,644
Subordinate Loans [Member] \| Various / Diversified [Member] \| Minimum [Member]
Mortgage Loans on Real Estate [Line Items]
Interest Payment Rates	10.75%
Maximum Maturity Date	2020
Subordinate Loans [Member] \| Various / Diversified [Member] \| Maximum [Member]
Mortgage Loans on Real Estate [Line Items]
Interest Payment Rates	12.83%
Maximum Maturity Date	2022

Stated maturity date of the mortgage loan receivable on real estate, in CCYY format.

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- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

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The reference rate for the variable rate of the debt instrument, such as LIBOR or the US Treasury rate and the maturity of the reference rate used, such as three months or six months LIBOR.

+ References

No definition available.

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- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
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- Definition

The carrying amount of the outstanding balance due under the mortgage loan.

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
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- Definition

Amount of the contractual principal due at the origination of the mortgage loan (face amount).

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- Definition

Identifies the site (such as country, region, state, county or municipality) of the property or properties under mortgage.

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- Definition

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.

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No definition available.

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- Definition

Description of the contractual payment terms of the loan receivable including whether principal and interest is payable at a level or varying amount over the life of the loan, the amount of any balloon payment required at maturity, and any prepayment penalties.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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Amount of prior lien on the real estate that secures the mortgage loan.

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