Schedule IV - Mortgage Loans on Real Estate (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Fixed Interest Payment Rates	5.15%
Prior Liens	$ 801,796
Face Amount of Loans	16,652,824	$ 16,277,343	$ 14,293,970
Carrying Amount of Loans	16,572,715	16,164,801	14,191,200	$ 10,056,732
CECL reserve	(173,549)	0
Loans receivable, net	16,399,166	16,164,801	$ 14,191,200
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	12,868,151
Carrying Amount of Loans	12,803,901
Senior Mortgage Loans [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Face Amount of Loans	16,451,802
Carrying Amount of Loans	16,376,645
Subordinate Loans [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Prior Liens	801,796
Face Amount of Loans	201,022
Carrying Amount of Loans	196,070
Office [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Carrying Amount of Loans	9,980,799	9,946,055
Multifamily [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Carrying Amount of Loans	1,788,149	1,596,333
Hospitality [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Carrying Amount of Loans	2,295,255	2,199,220
Industrial [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Carrying Amount of Loans	673,912	603,917
Other [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Carrying Amount of Loans	748,840	921,459
Ireland [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Carrying Amount of Loans	$ 1,309,443	1,318,196
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	$ 1,317,846
Carrying Amount of Loans	1,309,443
Spain [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Carrying Amount of Loans	$ 1,247,162	1,231,061
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	I/O
Face Amount of Loans	$ 993,263
Carrying Amount of Loans	$ 990,668
Southeast [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	3.40%
Maturity Date	2023
Periodic Payment Terms	I/O
Face Amount of Loans	$ 619,194
Carrying Amount of Loans	$ 618,766
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	$ 6,970,825
Carrying Amount of Loans	$ 6,931,306
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
Face Amount of Loans	$ 1,198,907
Carrying Amount of Loans	$ 1,195,218
Diversified [Member] | Hospitality [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	$ 2,097,826
Carrying Amount of Loans	$ 2,089,177
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 & P/I
Face Amount of Loans	$ 1,540,549
Carrying Amount of Loans	$ 1,531,806
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	$ 515,018
Carrying Amount of Loans	$ 514,322
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
Periodic Payment Terms	I/O & P/I
Face Amount of Loans	$ 545,026
Carrying Amount of Loans	$ 542,072
Diversified [Member] | Various [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	5.75%
Periodic Payment Terms	I/O & P/I
Prior Liens		801,796
Face Amount of Loans		201,022
Carrying Amount of Loans		$ 196,070
Northeast [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	2.30%
Maturity Date	2024
Periodic Payment Terms	I/O
Face Amount of Loans	$ 653,348
Carrying Amount of Loans	$ 653,867
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	2021
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	1.86%
Maturity Date	2021
Minimum [Member] | Diversified [Member] | Hospitality [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	2.35%
Maturity Date	2021
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	2.50%
Maturity Date	2022
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	2021
Minimum [Member] | Diversified [Member] | Various [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] | 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] | Hospitality [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] | 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	2025
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	3.55%
Maturity Date	2024
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 [Member] | Subordinate Loans [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Interest Payment Rates	5.25%
Maturity Date	2025