Loans Receivable, Net - Property Type and Geographic Distribution of Properties Securing Loans in Portfolio (Detail) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2021 USD ($) SecurityLoan	Dec. 31, 2020 USD ($) SecurityLoan	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	121	120
Net Book Value	$ 17,060,102	$ 16,572,715
Total Loan Exposure	$ 18,033,021	$ 17,454,621
Percentage of Portfolio	100.00%	100.00%
CECL reserve	$ (172,100)	$ (173,549)	$ (112,694)	$ 0
Net book value of restructured loan	$ 16,888,002	$ 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	59	58
Net Book Value	$ 9,701,241	$ 9,834,509
Total Loan Exposure	$ 10,228,801	$ 10,303,895
Percentage of Portfolio	57.00%	58.00%
Hospitality [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Number of Loans | SecurityLoan	14	14
Net Book Value	$ 2,327,639	$ 2,295,255
Total Loan Exposure	$ 2,411,594	$ 2,369,454
Percentage of Portfolio	13.00%	14.00%
Multifamily [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Number of Loans | SecurityLoan	33	31
Net Book Value	$ 2,161,102	$ 1,788,149
Total Loan Exposure	$ 2,240,222	$ 1,862,667
Percentage of Portfolio	12.00%	11.00%
Industrial [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Number of Loans | SecurityLoan	4	6
Net Book Value	$ 668,848	$ 673,912
Total Loan Exposure	$ 673,558	$ 675,344
Percentage of Portfolio	4.00%	4.00%
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	$ 535,666	$ 538,702
Total Loan Exposure	$ 548,128	$ 551,243
Percentage of Portfolio	3.00%	3.00%
Life Sciences [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Number of Loans | SecurityLoan	2	1
Net Book Value	$ 454,860	$ 146,290
Total Loan Exposure	$ 460,516	$ 147,763
Percentage of Portfolio	3.00%	1.00%
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	$ 285,537	$ 301,566
Total Loan Exposure	$ 285,555	$ 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	$ 223,095	$ 245,492
Total Loan Exposure	$ 251,943	$ 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	$ 702,114	$ 748,840
Total Loan Exposure	$ 932,704	$ 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	$ 4,255,718	$ 4,050,732
Total Loan Exposure	$ 4,277,969	$ 4,069,712
Percentage of Portfolio	24.00%	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	24	27
Net Book Value	$ 2,628,773	$ 2,942,126
Total Loan Exposure	$ 3,142,128	$ 3,413,089
Percentage of Portfolio	17.00%	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	26	25
Net Book Value	$ 2,857,343	$ 2,624,701
Total Loan Exposure	$ 2,982,274	$ 2,707,080
Percentage of Portfolio	17.00%	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	$ 979,640	$ 973,702
Total Loan Exposure	$ 981,965	$ 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	10	9
Net Book Value	$ 645,254	$ 597,100
Total Loan Exposure	$ 647,988	$ 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	$ 15,408	$ 15,404
Total Loan Exposure	$ 15,413	$ 15,413
United States [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Number of Loans | SecurityLoan	94	94
Net Book Value	$ 11,382,136	$ 11,203,765
Total Loan Exposure	$ 12,047,737	$ 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	14	13
Net Book Value	$ 1,945,475	$ 1,816,901
Total Loan Exposure	$ 2,196,903	$ 2,066,390
Percentage of Portfolio	12.00%	12.00%
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	$ 1,259,112	$ 1,309,443
Total Loan Exposure	$ 1,265,417	$ 1,317,846
Percentage of Portfolio	7.00%	8.00%
Spain [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	$ 1,167,734	$ 1,247,162
Total Loan Exposure	$ 1,171,926	$ 1,252,080
Percentage of Portfolio	7.00%	7.00%
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	$ 246,348	$ 259,126
Total Loan Exposure	$ 247,429	$ 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	$ 66,954	$ 82,185
Total Loan Exposure	$ 67,079	$ 82,262
International [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Number of Loans | SecurityLoan	27	26
Net Book Value	$ 5,677,966	$ 5,368,950
Total Loan Exposure	$ 5,985,284	$ 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	$ 992,343	$ 654,133
Total Loan Exposure	$ 1,036,530	$ 695,455
Percentage of Portfolio	6.00%	4.00%