Schedule IV - Mortgage Loans on Real Estate (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Prior Liens	$ 688,521
Face Amount of Loans	16,277,343
Carrying Amount of Loans	16,164,801	$ 14,191,200	$ 10,056,732	$ 8,692,978
Senior Mortgage Loans Less Than 3% of the Carrying Amount of Total Loans [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Face Amount of Loans	12,474,400
Carrying Amount of Loans	12,385,652
Senior Mortgage Loans [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Face Amount of Loans	16,111,731
Carrying Amount of Loans	16,003,736
Subordinate Loans [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Prior Liens	688,521
Face Amount of Loans	165,612
Carrying Amount of Loans	$ 161,065
Ireland [Member] | Office [Member] | Borrower A [Member] | Senior Mortgage Loans in Excess of 3% of the Carrying Amount of Total Loans [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Debt instrument variable rate basis	LIBOR
Interest Payment Rates	2.50%
Maturity Date	2024
Periodic Payment Terms	I/O
Face Amount of Loans	$ 1,330,647
Carrying Amount of Loans	$ 1,318,196
Spain [Member] | Mixed Use [Member] | Borrower B [Member] | Senior Mortgage Loans in Excess of 3% of the Carrying Amount of Total Loans [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Debt instrument variable rate basis	LIBOR
Interest Payment Rates	3.15%
Maturity Date	2023
Periodic Payment Terms	P/I
Face Amount of Loans	$ 1,008,605
Carrying Amount of Loans	$ 1,004,542
Southeast [Member] | Office [Member] | Borrower C [Member] | Senior Mortgage Loans in Excess of 3% of the Carrying Amount of Total Loans [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Debt instrument variable rate basis	LIBOR
Interest Payment Rates	3.40%
Maturity Date	2023
Periodic Payment Terms	I/O
Face Amount of Loans	$ 711,738
Carrying Amount of Loans	$ 710,260
Diversified [Member] | Office [Member] | Senior Mortgage Loans Less Than 3% of the Carrying Amount of Total Loans [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Debt instrument variable rate basis	LIBOR
Fixed Interest Payment Rates	4.46%
Periodic Payment Terms	I/O & P/I
Face Amount of Loans	$ 7,024,891
Carrying Amount of Loans	$ 6,970,837
Diversified [Member] | Hotel [Member] | Senior Mortgage Loans Less Than 3% of the Carrying Amount of Total Loans [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Debt instrument variable rate basis	LIBOR
Periodic Payment Terms	I/O
Face Amount of Loans	$ 2,034,662
Carrying Amount of Loans	$ 2,021,629
Diversified [Member] | Multifamily [Member] | Senior Mortgage Loans Less Than 3% of the Carrying Amount of Total Loans [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Debt instrument variable rate basis	LIBOR
Periodic Payment Terms	I/O & P/I
Face Amount of Loans	$ 1,252,517
Carrying Amount of Loans	$ 1,248,641
Diversified [Member] | Mixed Use [Member] | Senior Mortgage Loans Less Than 3% of the Carrying Amount of Total Loans [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Debt instrument variable rate basis	LIBOR
Periodic Payment Terms	I/O
Face Amount of Loans	$ 784,219
Carrying Amount of Loans	$ 775,922
Diversified [Member] | Industrial [Member] | Senior Mortgage Loans Less Than 3% of the Carrying Amount of Total Loans [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Debt instrument variable rate basis	LIBOR
Periodic Payment Terms	I/O
Face Amount of Loans	$ 676,938
Carrying Amount of Loans	$ 671,844
Diversified [Member] | Other [Member] | Senior Mortgage Loans Less Than 3% of the Carrying Amount of Total Loans [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Debt instrument variable rate basis	LIBOR
Fixed Interest Payment Rates	5.15%
Periodic Payment Terms	I/O & P/I
Face Amount of Loans	$ 701,173
Carrying Amount of Loans	$ 696,779
Diversified [Member] | Various / Diversified [Member] | Subordinate Loans [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Debt instrument variable rate basis	LIBOR
Fixed Interest Payment Rates	5.74%
Periodic Payment Terms	I/O & P/I
Prior Liens	$ 688,521
Face Amount of Loans	165,612
Carrying Amount of Loans	$ 161,065
Northeast [Member] | Office [Member] | Borrower D [Member] | Senior Mortgage Loans in Excess of 3% of the Carrying Amount of Total Loans [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Debt instrument variable rate basis	LIBOR
Interest Payment Rates	2.30%
Maturity Date	2024
Periodic Payment Terms	I/O
Face Amount of Loans	$ 586,341
Carrying Amount of Loans	$ 585,086
Minimum [Member] | Diversified [Member] | Office [Member] | Senior Mortgage Loans Less Than 3% of the Carrying Amount of Total Loans [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Interest Payment Rates	2.40%
Maturity Date	2020
Minimum [Member] | Diversified [Member] | Hotel [Member] | Senior Mortgage Loans Less Than 3% of the Carrying Amount of Total Loans [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Interest Payment Rates	2.75%
Maturity Date	2020
Minimum [Member] | Diversified [Member] | Multifamily [Member] | Senior Mortgage Loans Less Than 3% of the Carrying Amount of Total Loans [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Interest Payment Rates	2.60%
Maturity Date	2020
Minimum [Member] | Diversified [Member] | Mixed Use [Member] | Senior Mortgage Loans Less Than 3% of the Carrying Amount of Total Loans [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Interest Payment Rates	2.75%
Maturity Date	2021
Minimum [Member] | Diversified [Member] | Industrial [Member] | Senior Mortgage Loans Less Than 3% of the Carrying Amount of Total Loans [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Interest Payment Rates	2.60%
Maturity Date	2022
Minimum [Member] | Diversified [Member] | Other [Member] | Senior Mortgage Loans Less Than 3% of the Carrying Amount of Total Loans [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Interest Payment Rates	3.10%
Maturity Date	2020
Minimum [Member] | Diversified [Member] | Various / Diversified [Member] | Subordinate Loans [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Interest Payment Rates	3.12%
Maturity Date	2022
Maximum [Member] | Diversified [Member] | Office [Member] | Senior Mortgage Loans Less Than 3% of the Carrying Amount of Total Loans [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Interest Payment Rates	8.50%
Maturity Date	2026
Maximum [Member] | Diversified [Member] | Hotel [Member] | Senior Mortgage Loans Less Than 3% of the Carrying Amount of Total Loans [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Interest Payment Rates	3.75%
Maturity Date	2025
Maximum [Member] | Diversified [Member] | Multifamily [Member] | Senior Mortgage Loans Less Than 3% of the Carrying Amount of Total Loans [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Interest Payment Rates	5.00%
Maturity Date	2025
Maximum [Member] | Diversified [Member] | Mixed Use [Member] | Senior Mortgage Loans Less Than 3% of the Carrying Amount of Total Loans [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Interest Payment Rates	4.10%
Maturity Date	2024
Maximum [Member] | Diversified [Member] | Industrial [Member] | Senior Mortgage Loans Less Than 3% of the Carrying Amount of Total Loans [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Interest Payment Rates	3.85%
Maturity Date	2025
Maximum [Member] | Diversified [Member] | Other [Member] | Senior Mortgage Loans Less Than 3% of the Carrying Amount of Total Loans [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Interest Payment Rates	5.00%
Maturity Date	2026
Maximum [Member] | Diversified [Member] | Various / Diversified [Member] | Subordinate Loans [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Interest Payment Rates	5.28%
Maturity Date	2025