Loans Receivable, Net - Property Type and Geographic Distribution of Properties Securing Loans in Portfolio (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($) SecurityLoan	Dec. 31, 2018 USD ($) SecurityLoan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	128	125
Net book value	$ 16,164,801	$ 14,191,200
Total Loan Exposure	$ 16,965,864	$ 14,740,844
Percentage of Portfolio	100.00%	100.00%
Office [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	63	55
Net book value	$ 9,946,055	$ 7,104,842
Total Loan Exposure	$ 10,266,567	$ 7,164,466
Percentage of Portfolio	61.00%	49.00%
Hotel [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	14	18
Net book value	$ 2,199,220	$ 2,591,565
Total Loan Exposure	$ 2,281,718	$ 2,673,763
Percentage of Portfolio	13.00%	18.00%
Multifamily [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	36	34
Net book value	$ 1,596,333	$ 2,193,699
Total Loan Exposure	$ 1,642,664	$ 2,206,740
Percentage of Portfolio	10.00%	15.00%
Industrial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	5	5
Net book value	$ 603,917	$ 680,808
Total Loan Exposure	$ 607,423	$ 685,776
Percentage of Portfolio	4.00%	5.00%
Retail [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	3	4
Net book value	$ 373,045	$ 451,099
Total Loan Exposure	$ 381,040	$ 452,900
Percentage of Portfolio	2.00%	3.00%
Condominium [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	1	4
Net book value	$ 232,778	$ 304,545
Total Loan Exposure	$ 234,260	$ 368,104
Percentage of Portfolio	1.00%	2.00%
Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	4	3
Net book value	$ 921,459	$ 586,169
Total Loan Exposure	$ 1,259,696	$ 909,052
Percentage of Portfolio	7.00%	6.00%
Self-Storage [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	2	2
Net book value	$ 291,994	$ 278,473
Total Loan Exposure	$ 292,496	$ 280,043
Percentage of Portfolio	2.00%	2.00%
United States Northeast [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	25	32
Net book value	$ 3,789,477	$ 4,322,114
Total Loan Exposure	$ 3,815,580	$ 4,359,938
Percentage of Portfolio	22.00%	31.00%
United States West [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	30	29
Net book value	$ 3,143,323	$ 3,137,072
Total Loan Exposure	$ 3,451,914	$ 3,222,706
Percentage of Portfolio	20.00%	22.00%
United States Southeast [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	23	19
Net book value	$ 2,321,444	$ 2,258,033
Total Loan Exposure	$ 2,334,852	$ 2,271,664
Percentage of Portfolio	14.00%	15.00%
United States Midwest [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	10	9
Net book value	$ 1,174,581	$ 1,161,637
Total Loan Exposure	$ 1,180,240	$ 1,170,619
Percentage of Portfolio	7.00%	8.00%
United States Southwest [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	11	13
Net book value	$ 464,989	$ 478,665
Total Loan Exposure	$ 467,532	$ 481,745
Percentage of Portfolio	3.00%	3.00%
United States Northwest [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	3	4
Net book value	$ 52,891	$ 238,844
Total Loan Exposure	$ 52,989	$ 239,872
Percentage of Portfolio		2.00%
United States [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	102	106
Net book value	$ 10,946,705	$ 11,596,365
Total Loan Exposure	$ 11,303,107	$ 11,746,544
Percentage of Portfolio	66.00%	81.00%
United Kingdom [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	13	7
Net book value	$ 1,738,536	$ 754,299
Total Loan Exposure	$ 2,102,501	$ 1,094,663
Percentage of Portfolio	12.00%	7.00%
Spain [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	2	1
Net book value	$ 1,231,061	$ 1,124,174
Total Loan Exposure	$ 1,237,809	$ 1,131,334
Percentage of Portfolio	7.00%	8.00%
Canada [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	3	5
Net book value	$ 77,656	$ 316,268
Total Loan Exposure	$ 77,953	$ 313,229
Percentage of Portfolio	1.00%	2.00%
Australia [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	3	3
Net book value	$ 360,047	$ 310,372
Total Loan Exposure	$ 361,763	$ 312,893
Percentage of Portfolio	2.00%	2.00%
Belgium [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	1	1
Net book value	$ 86,807	$ 70,621
Total Loan Exposure	$ 87,201	$ 71,007
Percentage of Portfolio	1.00%
Germany [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	1	1
Net book value	$ 195,081	$ 11,585
Total Loan Exposure	$ 251,020	$ 63,637
Percentage of Portfolio	1.00%
Netherlands [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan		1
Net book value		$ 7,516
Total Loan Exposure		$ 7,537
International [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	26	19
Net book value	$ 5,218,096	$ 2,594,835
Total Loan Exposure	$ 5,662,757	$ 2,994,300
Percentage of Portfolio	34.00%	19.00%
Ireland [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	1
Net book value	$ 1,318,196
Total Loan Exposure	$ 1,330,647
Percentage of Portfolio	8.00%
France [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	1
Net book value	$ 31,972
Total Loan Exposure	$ 32,966
Percentage of Portfolio	1.00%
Italy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	1
Net book value	$ 178,740
Total Loan Exposure	$ 180,897
Percentage of Portfolio	1.00%