Loans Receivable, Net - Property Type and Geographic Distribution of Properties Securing Loans in Portfolio (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2020 USD ($) SecurityLoan	Dec. 31, 2019 USD ($) SecurityLoan	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Number of Loans | SecurityLoan	120	128
Net Book Value	$ 16,572,715	$ 16,164,801	$ 14,191,200	$ 10,056,732
Total Loan Exposure	$ 17,454,621	$ 16,965,864
Percentage of Portfolio	100.00%	100.00%
CECL reserve	$ (173,549)	$ 0
Net book value of restructured loan	$ 16,399,166	$ 16,164,801	$ 14,191,200
Office [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Number of Loans | SecurityLoan	59	63
Net Book Value	$ 9,980,799	$ 9,946,055
Total Loan Exposure	$ 10,451,658	$ 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	$ 2,295,255	$ 2,199,220
Total Loan Exposure	$ 2,369,454	$ 2,281,718
Percentage of Portfolio	14.00%	13.00%
Multifamily [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Number of Loans | SecurityLoan	31	36
Net Book Value	$ 1,788,149	$ 1,596,333
Total Loan Exposure	$ 1,862,667	$ 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	6	5
Net Book Value	$ 673,912	$ 603,917
Total Loan Exposure	$ 675,344	$ 607,423
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	3
Net Book Value	$ 538,702	$ 373,045
Total Loan Exposure	$ 551,243	$ 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	$ 301,566	$ 291,994
Total Loan Exposure	$ 301,491	$ 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	$ 245,492	$ 232,778
Total Loan Exposure	$ 264,162	$ 234,260
Percentage of Portfolio	2.00%	1.00%
Other [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Number of Loans | SecurityLoan	2	4
Net Book Value	$ 748,840	$ 921,459
Total Loan Exposure	$ 978,602	$ 1,259,696
Percentage of Portfolio	5.00%	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	24	25
Net Book Value	$ 4,050,732	$ 3,789,477
Total Loan Exposure	$ 4,069,712	$ 3,815,580
Percentage of Portfolio	23.00%	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	27	30
Net Book Value	$ 2,942,126	$ 3,143,323
Total Loan Exposure	$ 3,413,089	$ 3,451,914
Percentage of Portfolio	20.00%	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	$ 2,624,701	$ 2,321,444
Total Loan Exposure	$ 2,707,080	$ 2,334,852
Percentage of Portfolio	16.00%	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	8	10
Net Book Value	$ 973,702	$ 1,174,581
Total Loan Exposure	$ 976,693	$ 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	9	11
Net Book Value	$ 597,100	$ 464,989
Total Loan Exposure	$ 598,813	$ 467,532
Percentage of Portfolio	3.00%	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	$ 15,404	$ 52,891
Total Loan Exposure	$ 15,413	$ 52,989
Percentage of Portfolio	0.00%	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	94	102
Net Book Value	$ 11,203,765	$ 10,946,705
Total Loan Exposure	$ 11,780,800	$ 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	$ 1,816,901	$ 1,738,536
Total Loan Exposure	$ 2,066,390	$ 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	$ 1,309,443	$ 1,318,196
Total Loan Exposure	$ 1,317,846	$ 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	$ 1,247,162	$ 1,231,061
Total Loan Exposure	$ 1,252,080	$ 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	$ 214,795	$ 195,081
Total Loan Exposure	$ 252,816	$ 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	$ 259,126	$ 360,047
Total Loan Exposure	$ 259,788	$ 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	$ 203,982	$ 178,740
Total Loan Exposure	$ 205,514	$ 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	$ 105,300
Total Loan Exposure	$ 106,274
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	$ 94,757	$ 86,807
Total Loan Exposure	$ 94,936	$ 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	$ 82,185	$ 77,656
Total Loan Exposure	$ 82,262	$ 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	$ 35,299	$ 31,972
Total Loan Exposure	$ 35,915	$ 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	$ 5,368,950	$ 5,218,096
Total Loan Exposure	$ 5,673,821	$ 5,662,757
Percentage of Portfolio	32.00%	34.00%