Investments in Real Estate Debt - Summary of Investments in Real Estate Debt (Detail) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2020 USD ($) Position	Dec. 31, 2019 USD ($) Position
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Number of Positions | Position	241	203
Weighted Average Coupon	3.20%	4.40%
Weighted Average Maturity Date	Jun. 29, 2025	Sep. 21, 2025
Cost Basis	$ 5,297,531	$ 4,493,288
Fair Value	$ 4,965,677	$ 4,523,260
Total Real Estate Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Number of Positions | Position	230	195
Weighted Average Coupon	3.10%	4.20%
Weighted Average Maturity Date	Dec. 18, 2025	Oct. 15, 2025
Cost Basis	$ 4,600,415	$ 4,185,744
Fair Value	$ 4,255,432	$ 4,216,055
Total Real Estate Loans [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Number of Positions | Position	11	8
Weighted Average Maturity Date	Aug. 31, 2022	Oct. 16, 2024
Face Amount/ Notional	$ 733,283	$ 309,989
Cost Basis	697,116	307,544
Fair Value	$ 710,245	$ 307,205
Total Real Estate Loans [Member] | One-Month U.S. Dollar-Denominated London Interbank Offer Rate [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Weighted Average Coupon	3.70%	4.60%
Commercial Mortgage Backed Securities Floating
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Number of Positions | Position	138	122
Weighted Average Maturity Date	Jan. 05, 2025	Jan. 29, 2025
Face Amount/ Notional	$ 3,212,846	$ 2,907,080
Cost Basis	3,193,384	2,899,556
Fair Value	$ 2,914,116	$ 2,906,952
Commercial Mortgage Backed Securities Floating | One-Month U.S. Dollar-Denominated London Interbank Offer Rate [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Weighted Average Coupon	2.70%	2.70%
Commercial Mortgage Backed Securities Fixed
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Number of Positions | Position	54	43
Weighted Average Coupon	4.00%	4.20%
Weighted Average Maturity Date	Apr. 18, 2028	Jun. 26, 2027
Face Amount/ Notional	$ 1,022,626	$ 850,738
Cost Basis	991,867	829,403
Fair Value	$ 933,104	$ 831,970
Commercial Mortgage Backed Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Number of Positions | Position	5	5
Weighted Average Coupon	2.30%	2.30%
Weighted Average Maturity Date	Oct. 10, 2026	Oct. 02, 2026
Face Amount/ Notional	$ 2,258,809	$ 2,261,480
Cost Basis	21,748	23,564
Fair Value	$ 21,726	$ 23,547
Mezzanine Loan [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Number of Positions | Position	1	1
Weighted Average Maturity Date	Dec. 15, 2024	Dec. 15, 2024
Face Amount/ Notional	$ 134,750	$ 134,750
Cost Basis	134,338	134,078
Fair Value	$ 119,144	$ 134,076
Mezzanine Loan [Member] | One-Month U.S. Dollar-Denominated London Interbank Offer Rate [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Weighted Average Coupon	6.90%	6.90%
Commercial Mortgage Backed Securities Zero Coupon [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Number of Positions | Position	4	4
Weighted Average Maturity Date	Jan. 02, 2027	Dec. 30, 2026
Face Amount/ Notional	$ 236,090	$ 236,090
Cost Basis	134,960	127,219
Fair Value	$ 136,476	$ 136,027
Term Loans [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Number of Positions | Position	10	7
Weighted Average Maturity Date	Mar. 15, 2022	Aug. 30, 2024
Face Amount/ Notional	$ 598,533	$ 175,239
Cost Basis	562,778	173,466
Fair Value	$ 591,101	$ 173,129
Term Loans [Member] | One-Month U.S. Dollar-Denominated London Interbank Offer Rate [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Weighted Average Coupon	3.00%	2.90%
Residential Mortgage Backed Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Number of Positions | Position	14	9
Weighted Average Coupon	4.40%	4.30%
Weighted Average Maturity Date	Dec. 09, 2031	Jul. 09, 2028
Face Amount/ Notional	$ 28,462	$ 29,315
Cost Basis	28,635	29,506
Fair Value	$ 27,695	$ 29,448
Corporate Bond [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Number of Positions | Position	15	12
Weighted Average Coupon	5.00%	5.20%
Weighted Average Maturity Date	Feb. 28, 2027	Feb. 16, 2027
Face Amount/ Notional	$ 231,249	$ 276,302
Cost Basis	229,821	276,496
Fair Value	$ 222,315	$ 288,111