Investments in Real Estate-Related Securities and Loans - Summary of Investments in Real Estate-Related Securities and Loans (Detail) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2019 USD ($) Position	Dec. 31, 2018 USD ($) Position
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Number of Positions | Position	182	102
Cost Basis	$ 3,760,776	$ 2,281,416
Fair Value	$ 3,804,309	$ 2,259,913
Commercial Mortgage Backed Securities Floating [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Number of Positions | Position	118
Cost Basis	$ 2,685,393
Fair Value	$ 2,693,513
Commercial Mortgage Backed Securities Fixed [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Number of Positions | Position	33
Cost Basis	$ 501,097
Fair Value	$ 515,532
Commercial Mortgage Backed Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Number of Positions | Position	5	4
Cost Basis	$ 24,428	$ 12,374
Fair Value	$ 24,432	$ 12,373
Term Loans [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Number of Positions | Position	9
Cost Basis	$ 162,641
Fair Value	$ 162,555
Commercial Mortgage Backed Securities Zero Coupon [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Number of Positions | Position	4
Cost Basis	$ 124,723
Fair Value	$ 135,405
Corporate Bond [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Number of Positions | Position	11
Cost Basis	$ 255,805
Fair Value	$ 266,160
Commercial Mortgage Backed Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Number of Positions | Position		95
Cost Basis		$ 2,135,498
Fair Value		$ 2,114,910
Hospitality, Multifamily, Diversified [Member] | Commercial Mortgage Backed Securities Fixed [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Number of Positions | Position	9
Weighted Average Maturity Date	Dec. 05, 2026
Face Amount/ Notional	$ 157,129
Cost Basis	153,458
Fair Value	$ 155,941
Hospitality, Multifamily, Diversified [Member] | One-Month U.S. Dollar-Denominated London Interbank Offer Rate [Member] | Commercial Mortgage Backed Securities Fixed [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Weighted Average Coupon	4.50%
Multifamily [Member] | Commercial Mortgage Backed Securities Floating [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Number of Positions | Position	13
Weighted Average Maturity Date	Jun. 13, 2026
Face Amount/ Notional	$ 103,345
Cost Basis	103,782
Fair Value	$ 103,705
Multifamily [Member] | Commercial Mortgage Backed Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Number of Positions | Position	1
Weighted Average Maturity Date	Jan. 05, 2029
Face Amount/ Notional	$ 42,024
Cost Basis	12,140
Fair Value	$ 12,129
Multifamily [Member] | Commercial Mortgage Backed Securities Zero Coupon [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Number of Positions | Position	3	2
Weighted Average Maturity Date	Apr. 12, 2027	Mar. 02, 2027
Face Amount/ Notional	$ 208,817	$ 166,793
Cost Basis	104,403	80,892
Fair Value	$ 114,699	$ 81,875
Multifamily [Member] | Commercial Mortgage Backed Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Number of Positions | Position		7
Weighted Average Maturity Date		Jun. 13, 2026
Face Amount/ Notional		$ 35,442
Cost Basis		34,876
Fair Value		$ 34,951
Multifamily [Member] | One-Month U.S. Dollar-Denominated London Interbank Offer Rate [Member] | Commercial Mortgage Backed Securities Floating [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Weighted Average Coupon	2.47%
Multifamily [Member] | One-Month U.S. Dollar-Denominated London Interbank Offer Rate [Member] | Commercial Mortgage Backed Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Weighted Average Coupon	4.70%
Multifamily [Member] | One-Month U.S. Dollar-Denominated London Interbank Offer Rate [Member] | Commercial Mortgage Backed Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Weighted Average Coupon		1.99%
Diversified [Member] | Term Loans [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Number of Positions | Position	1
Weighted Average Maturity Date	Feb. 06, 2022
Face Amount/ Notional	$ 47,132
Cost Basis	46,073
Fair Value	$ 45,954
Diversified [Member] | One-Month U.S. Dollar-Denominated London Interbank Offer Rate [Member] | Term Loans [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Weighted Average Coupon	1.70%
A Credit Rating [Member] | Hospitality, Industrial, Retail, Diversified [Member] | Commercial Mortgage Backed Securities Floating [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Number of Positions | Position	6
Weighted Average Maturity Date	Nov. 07, 2024
Face Amount/ Notional	$ 275,883
Cost Basis	276,237
Fair Value	$ 277,259
A Credit Rating [Member] | Hospitality, Industrial, Retail, Diversified [Member] | One-Month U.S. Dollar-Denominated London Interbank Offer Rate [Member] | Commercial Mortgage Backed Securities Floating [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Weighted Average Coupon	2.05%
A Credit Rating [Member] | Multifamily [Member] | Commercial Mortgage Backed Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Number of Positions | Position	1	1
Weighted Average Maturity Date	May 02, 2025	Jul. 25, 2025
Face Amount/ Notional	$ 194,399	$ 194,399
Cost Basis	939	1,001
Fair Value	$ 940	$ 1,001
A Credit Rating [Member] | Multifamily [Member] | One-Month U.S. Dollar-Denominated London Interbank Offer Rate [Member] | Commercial Mortgage Backed Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Weighted Average Coupon	0.10%	0.10%
A Credit Rating [Member] | Hospitality, Industrial, Retail [Member] | Commercial Mortgage Backed Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Number of Positions | Position		3
Weighted Average Maturity Date		Mar. 10, 2023
Face Amount/ Notional		$ 89,165
Cost Basis		89,184
Fair Value		$ 88,358
A Credit Rating [Member] | Hospitality, Industrial, Retail [Member] | One-Month U.S. Dollar-Denominated London Interbank Offer Rate [Member] | Commercial Mortgage Backed Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Weighted Average Coupon		1.81%
B Credit Rating [Member] | Hospitality, Multifamily, Diversified [Member] | Commercial Mortgage Backed Securities Fixed [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Number of Positions | Position	5
Weighted Average Maturity Date	Jun. 03, 2026
Face Amount/ Notional	$ 132,950
Cost Basis	131,675
Fair Value	$ 133,962
B Credit Rating [Member] | Hospitality, Multifamily, Diversified [Member] | One-Month U.S. Dollar-Denominated London Interbank Offer Rate [Member] | Commercial Mortgage Backed Securities Fixed [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Weighted Average Coupon	4.50%
B Credit Rating [Member] | Hospitality, Multifamily, Office [Member] | Commercial Mortgage Backed Securities Floating [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Number of Positions | Position	22
Weighted Average Maturity Date	Nov. 02, 2024
Face Amount/ Notional	$ 467,437
Cost Basis	466,423
Fair Value	$ 468,201
B Credit Rating [Member] | Hospitality, Multifamily, Office [Member] | Commercial Mortgage Backed Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Number of Positions | Position		21
Weighted Average Maturity Date		Sep. 19, 2024
Face Amount/ Notional		$ 496,383
Cost Basis		495,095
Fair Value		$ 490,019
B Credit Rating [Member] | Hospitality, Multifamily, Office [Member] | One-Month U.S. Dollar-Denominated London Interbank Offer Rate [Member] | Commercial Mortgage Backed Securities Floating [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Weighted Average Coupon	3.44%
B Credit Rating [Member] | Hospitality, Multifamily, Office [Member] | One-Month U.S. Dollar-Denominated London Interbank Offer Rate [Member] | Commercial Mortgage Backed Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Weighted Average Coupon		3.56%
B Credit Rating [Member] | Hospitality Diversified [Member] | Term Loans [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Number of Positions | Position	6
Weighted Average Maturity Date	Jan. 16, 2025
Face Amount/ Notional	$ 62,038
Cost Basis	61,486
Fair Value	$ 61,469
B Credit Rating [Member] | Hospitality Diversified [Member] | One-Month U.S. Dollar-Denominated London Interbank Offer Rate [Member] | Term Loans [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Weighted Average Coupon	3.70%
B Credit Rating [Member] | Hospitality, Multifamily [Member] | Corporate Bond [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Number of Positions | Position	2
Weighted Average Maturity Date	May 21, 2025
Face Amount/ Notional	$ 31,170
Cost Basis	31,394
Fair Value	$ 32,145
B Credit Rating [Member] | Hospitality, Multifamily [Member] | One-Month U.S. Dollar-Denominated London Interbank Offer Rate [Member] | Corporate Bond [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Weighted Average Coupon	6.20%
BB Credit Rating [Member] | Hospitality, Multifamily, Diversified [Member] | Corporate Bond [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Number of Positions | Position	9
Weighted Average Maturity Date	Dec. 08, 2026
Face Amount/ Notional	$ 225,165
Cost Basis	224,411
Fair Value	$ 234,015
BB Credit Rating [Member] | Hospitality, Multifamily, Diversified [Member] | One-Month U.S. Dollar-Denominated London Interbank Offer Rate [Member] | Corporate Bond [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Weighted Average Coupon	5.50%
BB Credit Rating [Member] | Hospitality, Industrial, Multifamily,Office, Diversified, Other [Member] | Commercial Mortgage Backed Securities Floating [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Number of Positions | Position	43
Weighted Average Maturity Date	Nov. 12, 2024
Face Amount/ Notional	$ 1,027,601
Cost Basis	1,028,981
Fair Value	$ 1,032,524
BB Credit Rating [Member] | Hospitality, Industrial, Multifamily,Office, Diversified, Other [Member] | One-Month U.S. Dollar-Denominated London Interbank Offer Rate [Member] | Commercial Mortgage Backed Securities Floating [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Weighted Average Coupon	2.83%
BB Credit Rating [Member] | Multifamily [Member] | Residential Mortgage Backed Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Number of Positions | Position	2
Weighted Average Maturity Date	Jun. 11, 2022
Face Amount/ Notional	$ 6,689
Cost Basis	6,689
Fair Value	$ 6,712
BB Credit Rating [Member] | Multifamily [Member] | Commercial Mortgage Backed Securities Zero Coupon [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Number of Positions | Position	1
Weighted Average Maturity Date	Apr. 19, 2025
Face Amount/ Notional	$ 27,273
Cost Basis	20,320
Fair Value	$ 20,706
BB Credit Rating [Member] | Multifamily [Member] | One-Month U.S. Dollar-Denominated London Interbank Offer Rate [Member] | Residential Mortgage Backed Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Weighted Average Coupon	4.20%
BB Credit Rating [Member] | Hospitality [Member] | Corporate Bond [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Number of Positions | Position		1
Weighted Average Maturity Date		Sep. 15, 2026
Face Amount/ Notional		$ 52,652
Cost Basis		52,652
Fair Value		$ 50,755
BB Credit Rating [Member] | Hospitality [Member] | One-Month U.S. Dollar-Denominated London Interbank Offer Rate [Member] | Corporate Bond [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Weighted Average Coupon		6.50%
BB Credit Rating [Member] | Diversified [Member] | Term Loans [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Number of Positions | Position	1
Weighted Average Maturity Date	May 07, 2026
Face Amount/ Notional	$ 54,291
Cost Basis	54,042
Fair Value	$ 54,087
BB Credit Rating [Member] | Diversified [Member] | One-Month U.S. Dollar-Denominated London Interbank Offer Rate [Member] | Term Loans [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Weighted Average Coupon	2.75%
BB Credit Rating [Member] | Hospitality, Multifamily, Office, Diversified [Member] | Commercial Mortgage Backed Securities Fixed [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Number of Positions | Position	8
Weighted Average Maturity Date	May 20, 2026
Face Amount/ Notional	$ 122,964
Cost Basis	119,777
Fair Value	$ 123,802
BB Credit Rating [Member] | Hospitality, Multifamily, Office, Diversified [Member] | One-Month U.S. Dollar-Denominated London Interbank Offer Rate [Member] | Commercial Mortgage Backed Securities Fixed [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Weighted Average Coupon	4.10%
BB Credit Rating [Member] | Hospitality, Industrial, Multifamily, Office, Retail [Member] | Commercial Mortgage Backed Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Number of Positions | Position		38
Weighted Average Maturity Date		Sep. 04, 2024
Face Amount/ Notional		$ 941,240
Cost Basis		939,742
Fair Value		$ 930,411
BB Credit Rating [Member] | Hospitality, Industrial, Multifamily, Office, Retail [Member] | One-Month U.S. Dollar-Denominated London Interbank Offer Rate [Member] | Commercial Mortgage Backed Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Weighted Average Coupon		2.83%
BBB Credit Rating [Member] | Hospitality, Multifamily, Diversified [Member] | Commercial Mortgage Backed Securities Fixed [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Number of Positions | Position	11
Weighted Average Maturity Date	Nov. 06, 2027
Face Amount/ Notional	$ 99,103
Cost Basis	96,187
Fair Value	$ 101,827
BBB Credit Rating [Member] | Hospitality, Multifamily, Diversified [Member] | One-Month U.S. Dollar-Denominated London Interbank Offer Rate [Member] | Commercial Mortgage Backed Securities Fixed [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Weighted Average Coupon	4.00%
BBB Credit Rating [Member] | Hospitality, Multifamily, Office, Diversified, Other [Member] | Commercial Mortgage Backed Securities Floating [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Number of Positions | Position	34
Weighted Average Maturity Date	Nov. 05, 2024
Face Amount/ Notional	$ 808,188
Cost Basis	809,970
Fair Value	$ 811,824
BBB Credit Rating [Member] | Hospitality, Multifamily, Office, Diversified, Other [Member] | One-Month U.S. Dollar-Denominated London Interbank Offer Rate [Member] | Commercial Mortgage Backed Securities Floating [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Weighted Average Coupon	2.30%
BBB Credit Rating [Member] | Multifamily [Member] | Commercial Mortgage Backed Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Number of Positions | Position	1	1
Weighted Average Maturity Date	Jan. 05, 2028	May 25, 2028
Face Amount/ Notional	$ 225,803	$ 225,802
Cost Basis	1,397	1,414
Fair Value	$ 1,396	$ 1,415
BBB Credit Rating [Member] | Multifamily [Member] | One-Month U.S. Dollar-Denominated London Interbank Offer Rate [Member] | Commercial Mortgage Backed Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Weighted Average Coupon	0.10%	0.10%
BBB Credit Rating [Member] | Hospitality [Member] | Term Loans [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Number of Positions | Position	1
Weighted Average Maturity Date	Aug. 29, 2025
Face Amount/ Notional	$ 1,041
Cost Basis	1,040
Fair Value	$ 1,045
BBB Credit Rating [Member] | Hospitality [Member] | One-Month U.S. Dollar-Denominated London Interbank Offer Rate [Member] | Term Loans [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Weighted Average Coupon	2.25%
BBB Credit Rating [Member] | Hospitality, Industrial, Multifamily, Office [Member] | Commercial Mortgage Backed Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Number of Positions | Position		26
Weighted Average Maturity Date		Nov. 18, 2024
Face Amount/ Notional		$ 578,771
Cost Basis		576,601
Fair Value		$ 571,171
BBB Credit Rating [Member] | Hospitality, Industrial, Multifamily, Office [Member] | One-Month U.S. Dollar-Denominated London Interbank Offer Rate [Member] | Commercial Mortgage Backed Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Weighted Average Coupon		2.15%
AAA Credit Rating [Member] | Multifamily [Member] | Commercial Mortgage Backed Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Number of Positions | Position	2	2
Weighted Average Maturity Date	May 20, 2026	Mar. 12, 2027
Face Amount/ Notional	$ 1,800,104	$ 1,802,581
Cost Basis	9,952	9,959
Fair Value	$ 9,967	$ 9,957
AAA Credit Rating [Member] | Multifamily [Member] | One-Month U.S. Dollar-Denominated London Interbank Offer Rate [Member] | Commercial Mortgage Backed Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Weighted Average Coupon	0.10%	0.10%