Schedule IV - Mortgage Loans on Real Estate (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Prior Liens	$ 446,874
Face Amount of Loans	14,293,970	$ 10,108,226
Carrying Amount of Loans	14,191,200	$ 10,056,732	$ 8,692,978	$ 9,077,007
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	12,378,713
Carrying Amount of Loans	12,287,536
Senior Mortgage Loans [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Face Amount of Loans	14,192,344
Carrying Amount of Loans	14,090,480
Subordinate Loans [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Prior Liens	446,874
Face Amount of Loans	101,626
Carrying Amount of Loans	$ 100,720
Spain [Member] | Spanish Assets [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	3.15%
Maturity Date	2023
Periodic Payment Terms	P/I
Face Amount of Loans	$ 1,131,334
Carrying Amount of Loans	$ 1,124,174
Southeast [Member] | Office [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	3.40%
Maturity Date	2023
Periodic Payment Terms	I/O
Face Amount of Loans	$ 682,297
Carrying Amount of Loans	$ 678,770
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
Periodic Payment Terms	I/O & P/I
Face Amount of Loans	$ 6,424,993
Carrying Amount of Loans	$ 6,370,682
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 & P/I
Face Amount of Loans	$ 2,428,085
Carrying Amount of Loans	$ 2,412,775
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	$ 1,878,502
Carrying Amount of Loans	$ 1,869,375
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	$ 686,951
Carrying Amount of Loans	$ 679,344
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	$ 565,754
Carrying Amount of Loans	$ 561,473
Diversified [Member] | Retail [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	5.09%
Periodic Payment Terms	I/O & P/I
Face Amount of Loans	$ 133,210
Carrying Amount of Loans	$ 133,777
Diversified [Member] | Self-Storage [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	$ 125,039
Carrying Amount of Loans	$ 124,831
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	$ 98,580
Carrying Amount of Loans	$ 98,303
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	5.75%
Maturity Date	2022
Periodic Payment Terms	I/O & P/I
Prior Liens	$ 446,874
Face Amount of Loans	101,626
Carrying Amount of Loans	$ 100,720
Northeast [Member] | Condo / Northeast [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	$ 37,599
Carrying Amount of Loans	$ 36,976
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	2.50%
Fixed Interest Payment Rates	4.46%
Maturity Date	2019
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	2.70%
Maturity Date	2019
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	2.25%
Maturity Date	2019
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	2.90%
Maturity Date	2022
Minimum [Member] | Diversified [Member] | Retail [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	2019
Minimum [Member] | Diversified [Member] | Self-Storage [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	3.95%
Maturity Date	2020
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	4.00%
Fixed Interest Payment Rates	4.59%
Maturity Date	2019
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	4.75%
Minimum [Member] | Northeast [Member] | Condo / Northeast [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	4.15%
Maturity Date	2021
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	10.35%
Fixed Interest Payment Rates	5.29%
Maturity Date	2025
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	4.94%
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	3.20%
Maturity Date	2024
Maximum [Member] | Diversified [Member] | Retail [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	3.95%
Maturity Date	2021
Maximum [Member] | Diversified [Member] | Self-Storage [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]
Maturity Date	2021
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]
Maturity Date	2023
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	5.25%
Maximum [Member] | Northeast [Member] | Condo / Northeast [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	2023