Investments in Real Estate-Related Securities and Loans - Summary of Investments in Real Estate-Related Securities and Loans (Detail) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019 USD ($) Position	Dec. 31, 2018 USD ($) Position
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Cost Basis	$ 1,067,092	$ 1,082,796
Fair Value	$ 2,386,131	$ 2,259,913
Corporate Bond [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Number of Positions | Position	118	102
Cost Basis	$ 2,376,599	$ 2,281,416
Fair Value	$ 2,386,131	$ 2,259,913
Multifamily [Member] | Commercial Mortgage Backed Securities Floating [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Number of Positions | Position	8
Weighted Average Maturity Date	Aug. 12, 2025
Face Amount/ Notional	$ 41,574
Cost Basis	41,326
Fair Value	$ 41,369
Multifamily [Member] | Commercial Mortgage Backed Securities Fixed [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Number of Positions | Position	3
Weighted Average Maturity Date	Oct. 17, 2027
Face Amount/ Notional	$ 16,998
Cost Basis	15,762
Fair Value	$ 16,354
Multifamily [Member] | Zero Coupon [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Number of Positions | Position	2	2
Weighted Average Maturity Date	Dec. 01, 2026	Mar. 02, 2027
Face Amount/ Notional	$ 166,793	$ 166,793
Cost Basis	82,572	80,892
Fair Value	$ 87,600	$ 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	2.44%
Multifamily [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	4.30%
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%
Hospitality, Diversified [Member] | Corporate Bond [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Number of Positions | Position	2
Weighted Average Maturity Date	Jul. 07, 2026
Face Amount/ Notional	$ 57,329
Cost Basis	57,300
Fair Value	$ 59,715
Hospitality, 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	6.50%
A Credit Rating [Member] | Hospitality, Industrial, Retail [Member] | Commercial Mortgage Backed Securities Floating [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Number of Positions | Position	3
Weighted Average Maturity Date	Jan. 03, 2023
Face Amount/ Notional	$ 78,320
Cost Basis	78,337
Fair Value	$ 78,231
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 Floating [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Weighted Average Coupon	1.82%
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%
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	1,004	1,001
Fair Value	$ 1,004	$ 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%
BBB Credit Rating [Member] | Multifamily [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	Jul. 16, 2027
Face Amount/ Notional	$ 28,213
Cost Basis	26,256
Fair Value	$ 27,668
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	$ 225,802	$ 225,802
Cost Basis	1,417	1,414
Fair Value	$ 1,418	$ 1,415
BBB Credit Rating [Member] | Multifamily [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	4.20%
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, Industrial, Multifamily, Office [Member] | Commercial Mortgage Backed Securities Floating [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Number of Positions | Position	24
Weighted Average Maturity Date	Jun. 26, 2024
Face Amount/ Notional	$ 556,025
Cost Basis	555,630
Fair Value	$ 554,861
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 Floating [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Weighted Average Coupon	2.23%
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%
BB Credit Rating [Member] | Hospitality, Industrial, Multifamily, Office, Retail [Member] | Commercial Mortgage Backed Securities Floating [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Number of Positions | Position	40
Weighted Average Maturity Date	Aug. 07, 2024
Face Amount/ Notional	$ 978,540
Cost Basis	976,867
Fair Value	$ 977,019
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 Floating [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Weighted Average Coupon	2.81%
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%
BB Credit Rating [Member] | Hospitality [Member] | Commercial Mortgage Backed Securities Fixed [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Number of Positions | Position	1
Weighted Average Maturity Date	Sep. 22, 2022
Face Amount/ Notional	$ 24,613
Cost Basis	24,049
Fair Value	$ 24,850
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] | 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	$ 1,046
Cost Basis	1,044
Fair Value	$ 1,040
BB Credit Rating [Member] | Hospitality [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	4.40%
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] | 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%
B Credit Rating [Member] | Multifamily [Member] | Commercial Mortgage Backed Securities Fixed [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Number of Positions | Position	1
Weighted Average Maturity Date	Aug. 26, 2022
Face Amount/ Notional	$ 6,704
Cost Basis	6,687
Fair Value	$ 6,767
B Credit Rating [Member] | Multifamily [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	5.20%
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	Aug. 17, 2024
Face Amount/ Notional	$ 496,019
Cost Basis	494,747
Fair Value	$ 494,646
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	3.55%
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 [Member] | Term Loans [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Number of Positions | Position	2
Weighted Average Maturity Date	Oct. 02, 2024
Face Amount/ Notional	$ 3,809
Cost Basis	3,715
Fair Value	$ 3,702
B 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	3.90%
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	Apr. 19, 2026	Mar. 12, 2027
Face Amount/ Notional	$ 1,801,735	$ 1,802,581
Cost Basis	9,886	9,959
Fair Value	$ 9,887	$ 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%