Loans Receivable, Net - Property Type and Geographic Distribution of Properties Securing Loans in Portfolio (Detail) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 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	$ 14,755,072	$ 14,191,200
Total Loan Exposure	$ 15,382,934	$ 14,740,844
Percentage of Portfolio	100.00%	100.00%
Office [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	59	55
Net book value	$ 7,654,154	$ 7,104,842
Total Loan Exposure	$ 7,838,768	$ 7,164,466
Percentage of Portfolio	50.00%	49.00%
Hotel [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	17	18
Net book value	$ 2,367,575	$ 2,591,565
Total Loan Exposure	$ 2,444,738	$ 2,673,763
Percentage of Portfolio	16.00%	18.00%
Multifamily [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	37	34
Net book value	$ 2,272,220	$ 2,193,699
Total Loan Exposure	$ 2,312,100	$ 2,206,740
Percentage of Portfolio	15.00%	15.00%
Industrial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	5	5
Net book value	$ 700,751	$ 680,808
Total Loan Exposure	$ 704,098	$ 685,776
Percentage of Portfolio	5.00%	5.00%
Retail [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	3	4
Net book value	$ 381,451	$ 451,099
Total Loan Exposure	$ 388,138	$ 452,900
Percentage of Portfolio	3.00%	3.00%
Condominium [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	1	4
Net book value	$ 223,908	$ 304,545
Total Loan Exposure	$ 225,116	$ 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	$ 874,133	$ 586,169
Total Loan Exposure	$ 1,188,383	$ 909,052
Percentage of Portfolio	8.00%	6.00%
Self-Storage [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	2	2
Net book value	$ 280,880	$ 278,473
Total Loan Exposure	$ 281,593	$ 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	29	32
Net book value	$ 4,069,651	$ 4,322,114
Total Loan Exposure	$ 4,097,215	$ 4,359,938
Percentage of Portfolio	27.00%	31.00%
United States West [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	29	29
Net book value	$ 3,186,197	$ 3,137,072
Total Loan Exposure	$ 3,370,213	$ 3,222,706
Percentage of Portfolio	22.00%	22.00%
United States Southeast [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	19	19
Net book value	$ 2,212,190	$ 2,258,033
Total Loan Exposure	$ 2,222,654	$ 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	11	9
Net book value	$ 1,283,170	$ 1,161,637
Total Loan Exposure	$ 1,289,119	$ 1,170,619
Percentage of Portfolio	8.00%	8.00%
United States Southwest [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	13	13
Net book value	$ 626,893	$ 478,665
Total Loan Exposure	$ 630,498	$ 481,745
Percentage of Portfolio	4.00%	3.00%
United States Northwest [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	4	4
Net book value	$ 177,295	$ 238,844
Total Loan Exposure	$ 177,900	$ 239,872
Percentage of Portfolio	1.00%	2.00%
United States [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	105	106
Net book value	$ 11,555,396	$ 11,596,365
Total Loan Exposure	$ 11,787,599	$ 11,746,544
Percentage of Portfolio	76.00%	81.00%
United Kingdom [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	11	7
Net book value	$ 1,331,763	$ 754,299
Total Loan Exposure	$ 1,667,035	$ 1,094,663
Percentage of Portfolio	11.00%	7.00%
Spain [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	2	1
Net book value	$ 1,109,276	$ 1,124,174
Total Loan Exposure	$ 1,115,580	$ 1,131,334
Percentage of Portfolio	7.00%	8.00%
Canada [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	4	5
Net book value	$ 148,291	$ 316,268
Total Loan Exposure	$ 147,880	$ 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	$ 337,107	$ 310,372
Total Loan Exposure	$ 338,975	$ 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	$ 84,223	$ 70,621
Total Loan Exposure	$ 84,759	$ 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	$ 189,016	$ 11,585
Total Loan Exposure	$ 241,106	$ 63,637
Percentage of Portfolio	2.00%
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	23	19
Net book value	$ 3,199,676	$ 2,594,835
Total Loan Exposure	$ 3,595,335	$ 2,994,300
Percentage of Portfolio	24.00%	19.00%
Ireland [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	1