Investments in Real Estate-Related Securities - Summary of Investments in CMBS (Detail) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018 USD ($) Position	Dec. 31, 2017 USD ($) Position
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Cost Basis	$ 1,013,713	$ 623,149
Fair Value	$ 2,029,334	$ 915,742
Commercial Mortgage Backed Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Number of Positions | Position	93	37
Face Amount	$ 2,068,363	$ 913,903
Cost Basis	2,018,203	913,372
Fair Value	$ 2,029,334	$ 915,742
Multifamily, Hospitality [Member] | Commercial Mortgage Backed Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Number of Positions | Position	9
Weighted Average Maturity Date	May 27, 2026
Face Amount	$ 138,127
Cost Basis	83,925
Fair Value	$ 84,127
Multifamily [Member] | Commercial Mortgage Backed Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Number of Positions | Position	2	3
Weighted Average Maturity Date	May 25, 2028	Sep. 15, 2026
Face Amount		$ 11,749
Cost Basis	$ 7,448	11,749
Fair Value	$ 7,447	$ 11,737
One-Month U.S. Dollar-Denominated London Interbank Offer Rate [Member] | Multifamily, Hospitality [Member] | Commercial Mortgage Backed Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Weighted Average Coupon	1.22%
One-Month U.S. Dollar-Denominated London Interbank Offer Rate [Member] | Multifamily [Member] | Commercial Mortgage Backed Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Weighted Average Coupon		2.50%
BB Credit Rating [Member] | Hospitality, Office, Multifamily, Retail [Member] | Commercial Mortgage Backed Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Number of Positions | Position	38	15
Weighted Average Maturity Date	Sep. 11, 2024	Dec. 18, 2022
Face Amount	$ 949,082	$ 423,770
Cost Basis	947,436	423,658
Fair Value	$ 953,583	$ 424,419
BB Credit Rating [Member] | One-Month U.S. Dollar-Denominated London Interbank Offer Rate [Member] | Hospitality, Office, Multifamily, Retail [Member] | Commercial Mortgage Backed Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Weighted Average Coupon	2.70%	3.21%
BBB Credit Rating [Member] | Hospitality, Office, Multifamily, Industrial, Retail [Member] | Commercial Mortgage Backed Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Number of Positions | Position	24	9
Weighted Average Maturity Date	Oct. 07, 2024	May 11, 2022
Face Amount	$ 486,858	$ 194,013
Cost Basis	486,362	193,838
Fair Value	$ 488,141	$ 194,549
BBB Credit Rating [Member] | One-Month U.S. Dollar-Denominated London Interbank Offer Rate [Member] | Hospitality, Office, Multifamily, Industrial, Retail [Member] | Commercial Mortgage Backed Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Weighted Average Coupon	2.15%	2.28%
B Credit Rating [Member] | Hospitality, Office, Multifamily, Retail [Member] | Commercial Mortgage Backed Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Number of Positions | Position	20
Weighted Average Maturity Date	Sep. 16, 2024
Face Amount	$ 494,296
Cost Basis	493,032
Fair Value	$ 496,036
B Credit Rating [Member] | Hospitality, Office, Multifamily [Member] | Commercial Mortgage Backed Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Number of Positions | Position		10
Weighted Average Maturity Date		Jan. 27, 2024
Face Amount		$ 284,371
Cost Basis		284,127
Fair Value		$ 285,037
B Credit Rating [Member] | One-Month U.S. Dollar-Denominated London Interbank Offer Rate [Member] | Hospitality, Office, Multifamily, Retail [Member] | Commercial Mortgage Backed Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Weighted Average Coupon	3.57%
B Credit Rating [Member] | One-Month U.S. Dollar-Denominated London Interbank Offer Rate [Member] | Hospitality, Office, Multifamily [Member] | Commercial Mortgage Backed Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Weighted Average Coupon		4.05%