Loans Receivable, Net - Property Type and Geographic Distribution of Properties Securing Loans in Portfolio (Detail) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2021 USD ($) SecurityLoan	Dec. 31, 2020 USD ($) SecurityLoan	Mar. 31, 2021 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Number of Loans | SecurityLoan	124	120
Net Book Value	$ 17,436,843	$ 16,572,715
Total Loan Exposure	$ 18,528,773	$ 17,454,621
Percentage of Portfolio	100.00%	100.00%
CECL reserve	$ (128,945)	$ (173,549)	$ (172,100)	$ (178,050)	$ (112,694)	$ 0
Net book value of restructured loan	$ 17,307,898	$ 16,399,166
Office [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Number of Loans | SecurityLoan	58	58
Net Book Value	$ 9,183,532	$ 9,834,509
Total Loan Exposure	$ 9,799,741	$ 10,303,895
Percentage of Portfolio	53.00%	58.00%
Hospitality [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Number of Loans | SecurityLoan	15	14
Net Book Value	$ 2,497,689	$ 2,295,255
Total Loan Exposure	$ 2,593,256	$ 2,369,454
Percentage of Portfolio	14.00%	14.00%
Multifamily [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Number of Loans | SecurityLoan	34	31
Net Book Value	$ 2,458,186	$ 1,788,149
Total Loan Exposure	$ 2,542,024	$ 1,862,667
Percentage of Portfolio	14.00%	11.00%
Industrial [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Number of Loans | SecurityLoan	5	6
Net Book Value	$ 989,199	$ 673,912
Total Loan Exposure	$ 996,475	$ 675,344
Percentage of Portfolio	5.00%	4.00%
Life Sciences [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Number of Loans | SecurityLoan	3	1
Net Book Value	$ 559,794	$ 146,290
Total Loan Exposure	$ 566,806	$ 147,763
Percentage of Portfolio	3.00%	1.00%
Retail [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Number of Loans | SecurityLoan	4	4
Net Book Value	$ 538,439	$ 538,702
Total Loan Exposure	$ 551,221	$ 551,243
Percentage of Portfolio	3.00%	3.00%
Self-Storage [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Number of Loans | SecurityLoan	1	2
Net Book Value	$ 285,505	$ 301,566
Total Loan Exposure	$ 285,523	$ 301,491
Percentage of Portfolio	2.00%	2.00%
Condominium [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Number of Loans | SecurityLoan	2	2
Net Book Value	$ 226,698	$ 245,492
Total Loan Exposure	$ 265,484	$ 264,162
Percentage of Portfolio	1.00%	2.00%
Other [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Number of Loans | SecurityLoan	2	2
Net Book Value	$ 697,801	$ 748,840
Total Loan Exposure	$ 928,243	$ 978,602
Percentage of Portfolio	5.00%	5.00%
United States Northeast [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Number of Loans | SecurityLoan	25	24
Net Book Value	$ 4,428,506	$ 4,050,732
Total Loan Exposure	$ 4,452,355	$ 4,069,712
Percentage of Portfolio	25.00%	23.00%
United States West [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Number of Loans | SecurityLoan	25	27
Net Book Value	$ 2,788,728	$ 2,942,126
Total Loan Exposure	$ 3,373,399	$ 3,413,089
Percentage of Portfolio	18.00%	20.00%
United States Southeast [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Number of Loans | SecurityLoan	27	25
Net Book Value	$ 2,656,280	$ 2,624,701
Total Loan Exposure	$ 2,825,388	$ 2,707,080
Percentage of Portfolio	15.00%	16.00%
United States Midwest [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Number of Loans | SecurityLoan	8	8
Net Book Value	$ 964,681	$ 973,702
Total Loan Exposure	$ 967,089	$ 976,693
Percentage of Portfolio	5.00%	6.00%
United States Southwest [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Number of Loans | SecurityLoan	12	9
Net Book Value	$ 772,307	$ 597,100
Total Loan Exposure	$ 775,897	$ 598,813
Percentage of Portfolio	4.00%	3.00%
United States Northwest [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Number of Loans | SecurityLoan	1	1
Net Book Value	$ 15,412	$ 15,404
Total Loan Exposure	$ 15,413	$ 15,413
Percentage of Portfolio		0.00%
United States [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Number of Loans | SecurityLoan	98	94
Net Book Value	$ 11,625,914	$ 11,203,765
Total Loan Exposure	$ 12,409,541	$ 11,780,800
Percentage of Portfolio	67.00%	68.00%
United Kingdom [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Number of Loans | SecurityLoan	12	13
Net Book Value	$ 1,669,873	$ 1,816,901
Total Loan Exposure	$ 1,919,945	$ 2,066,390
Percentage of Portfolio	11.00%	12.00%
Spain [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Number of Loans | SecurityLoan	3	2
Net Book Value	$ 1,314,315	$ 1,247,162
Total Loan Exposure	$ 1,320,266	$ 1,252,080
Percentage of Portfolio	7.00%	7.00%
Ireland [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Number of Loans | SecurityLoan	1	1
Net Book Value	$ 1,289,458	$ 1,309,443
Total Loan Exposure	$ 1,295,372	$ 1,317,846
Percentage of Portfolio	7.00%	8.00%
Australia [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Number of Loans | SecurityLoan	2	2
Net Book Value	$ 242,987	$ 259,126
Total Loan Exposure	$ 243,797	$ 259,788
Percentage of Portfolio	1.00%	1.00%
Canada [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Number of Loans | SecurityLoan	2	3
Net Book Value	$ 68,430	$ 82,185
Total Loan Exposure	$ 68,486	$ 82,262
Percentage of Portfolio		0.00%
International [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Number of Loans | SecurityLoan	26	26
Net Book Value	$ 5,810,929	$ 5,368,950
Total Loan Exposure	$ 6,119,232	$ 5,673,821
Percentage of Portfolio	33.00%	32.00%
Other Europe [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Number of Loans | SecurityLoan	6	5
Net Book Value	$ 1,225,866	$ 654,133
Total Loan Exposure	$ 1,271,366	$ 695,455
Percentage of Portfolio	7.00%	4.00%