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, 2020 USD ($) SecurityLoan	Dec. 31, 2019 USD ($) SecurityLoan	Mar. 31, 2020 USD ($)	Dec. 31, 2018 USD ($)
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Number of Loans | SecurityLoan	128	128
Net Book Value	$ 16,339,403	$ 16,164,801
Total Loan Exposure	$ 17,174,244	$ 16,965,864		$ 16,164,801
Percentage of Portfolio	100.00%	100.00%
CECL reserve	$ (178,050)	$ 0	$ (112,694)
Net book value of restructured loan	$ 16,161,353	$ 16,164,801
Office [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Number of Loans | SecurityLoan	60	63
Net Book Value	$ 9,580,065	$ 9,946,055
Total Loan Exposure	$ 9,940,103	$ 10,266,567
Percentage of Portfolio	59.00%	61.00%
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	$ 2,220,051	$ 2,199,220
Total Loan Exposure	$ 2,295,799	$ 2,281,718
Percentage of Portfolio	13.00%	13.00%
Multifamily [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Number of Loans | SecurityLoan	36	36
Net Book Value	$ 1,881,529	$ 1,596,333
Total Loan Exposure	$ 1,947,388	$ 1,642,664
Percentage of Portfolio	11.00%	10.00%
Industrial [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Number of Loans | SecurityLoan	7	5
Net Book Value	$ 840,065	$ 603,917
Total Loan Exposure	$ 844,665	$ 607,423
Percentage of Portfolio	5.00%	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	3
Net Book Value	$ 533,088	$ 373,045
Total Loan Exposure	$ 544,682	$ 381,040
Percentage of Portfolio	3.00%	2.00%
Self-Storage [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	$ 289,329	$ 291,994
Total Loan Exposure	$ 289,441	$ 292,496
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	1
Net Book Value	$ 232,220	$ 232,778
Total Loan Exposure	$ 233,621	$ 234,260
Percentage of Portfolio	1.00%	1.00%
Other [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Number of Loans | SecurityLoan	3	4
Net Book Value	$ 763,056	$ 921,459
Total Loan Exposure	$ 1,078,545	$ 1,259,696
Percentage of Portfolio	6.00%	7.00%
United States Northeast [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Number of Loans | SecurityLoan	27	25
Net Book Value	$ 4,277,301	$ 3,789,477
Total Loan Exposure	$ 4,301,875	$ 3,815,580
Percentage of Portfolio	25.00%	22.00%
United States West [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Number of Loans | SecurityLoan	28	30
Net Book Value	$ 2,924,455	$ 3,143,323
Total Loan Exposure	$ 3,304,345	$ 3,451,914
Percentage of Portfolio	19.00%	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	25	23
Net Book Value	$ 2,363,782	$ 2,321,444
Total Loan Exposure	$ 2,376,630	$ 2,334,852
Percentage of Portfolio	14.00%	14.00%
United States Midwest [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Number of Loans | SecurityLoan	9	10
Net Book Value	$ 1,044,542	$ 1,174,581
Total Loan Exposure	$ 1,048,537	$ 1,180,240
Percentage of Portfolio	6.00%	7.00%
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	11
Net Book Value	$ 614,003	$ 464,989
Total Loan Exposure	$ 616,253	$ 467,532
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	3
Net Book Value	$ 15,515	$ 52,891
Total Loan Exposure	$ 15,530	$ 52,989
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	102	102
Net Book Value	$ 11,239,598	$ 10,946,705
Total Loan Exposure	$ 11,663,170	$ 11,303,107
Percentage of Portfolio	68.00%	66.00%
United Kingdom [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Number of Loans | SecurityLoan	13	13
Net Book Value	$ 1,658,666	$ 1,738,536
Total Loan Exposure	$ 1,997,241	$ 2,102,501
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	$ 1,323,243	$ 1,318,196
Total Loan Exposure	$ 1,333,139	$ 1,330,647
Percentage of Portfolio	8.00%	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	$ 1,208,597	$ 1,231,061
Total Loan Exposure	$ 1,214,209	$ 1,237,809
Percentage of Portfolio	7.00%	7.00%
Germany [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	$ 198,675	$ 195,081
Total Loan Exposure	$ 250,803	$ 251,020
Percentage of Portfolio	1.00%	1.00%
Australia [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	$ 232,376	$ 360,047
Total Loan Exposure	$ 233,425	$ 361,763
Percentage of Portfolio	1.00%	2.00%
Italy [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	$ 186,671	$ 178,740
Total Loan Exposure	$ 188,510	$ 180,897
Percentage of Portfolio	1.00%	1.00%
Netherlands [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Number of Loans | SecurityLoan	1
Net Book Value	$ 96,634
Total Loan Exposure	$ 97,731
Percentage of Portfolio	1.00%
Belgium [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	$ 87,222	$ 86,807
Total Loan Exposure	$ 87,304	$ 87,201
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	3	3
Net Book Value	$ 75,534	$ 77,656
Total Loan Exposure	$ 75,684	$ 77,953
Percentage of Portfolio	0.00%	1.00%
France [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	$ 32,187	$ 31,972
Total Loan Exposure	$ 33,028	$ 32,966
Percentage of Portfolio	0.00%	1.00%
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	$ 5,099,805	$ 5,218,096
Total Loan Exposure	$ 5,511,074	$ 5,662,757
Percentage of Portfolio	32.00%	34.00%