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, 2019 USD ($) SecurityLoan	Dec. 31, 2018 USD ($) SecurityLoan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	123	125
Net book value	$ 14,280,919	$ 14,191,200
Total Loan Exposure	$ 14,779,252	$ 14,740,844
Percentage of Portfolio	100.00%	100.00%
Office [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	54	55
Net book value	$ 7,205,141	$ 7,104,842
Total Loan Exposure	$ 7,258,497	$ 7,164,466
Percentage of Portfolio	48.00%	49.00%
Hotel [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	17	18
Net book value	$ 2,380,920	$ 2,591,565
Total Loan Exposure	$ 2,460,389	$ 2,673,763
Percentage of Portfolio	17.00%	18.00%
Multifamily [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	38	34
Net book value	$ 2,243,177	$ 2,193,699
Total Loan Exposure	$ 2,272,201	$ 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	$ 685,128	$ 680,808
Total Loan Exposure	$ 688,981	$ 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	$ 381,509	$ 451,099
Total Loan Exposure	$ 386,617	$ 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	$ 228,817	$ 304,545
Total Loan Exposure	$ 230,258	$ 368,104
Percentage of Portfolio	2.00%	2.00%
Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	3	3
Net book value	$ 874,237	$ 586,169
Total Loan Exposure	$ 1,199,331	$ 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	$ 281,990	$ 278,473
Total Loan Exposure	$ 282,978	$ 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	27	32
Net book value	$ 3,999,237	$ 4,322,114
Total Loan Exposure	$ 4,025,533	$ 4,359,938
Percentage of Portfolio	28.00%	31.00%
United States West [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	26	29
Net book value	$ 2,953,508	$ 3,137,072
Total Loan Exposure	$ 2,995,900	$ 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	21	19
Net book value	$ 2,409,038	$ 2,258,033
Total Loan Exposure	$ 2,420,751	$ 2,271,664
Percentage of Portfolio	16.00%	15.00%
United States Midwest [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	10	9
Net book value	$ 1,109,007	$ 1,161,637
Total Loan Exposure	$ 1,115,860	$ 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	14	13
Net book value	$ 546,008	$ 478,665
Total Loan Exposure	$ 548,794	$ 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	$ 175,039	$ 238,844
Total Loan Exposure	$ 175,770	$ 239,872
Percentage of Portfolio	1.00%	2.00%
United States [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	102	106
Net book value	$ 11,191,837	$ 11,596,365
Total Loan Exposure	$ 11,282,608	$ 11,746,544
Percentage of Portfolio	77.00%	81.00%
United Kingdom [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	11	7
Net book value	$ 1,270,547	$ 754,299
Total Loan Exposure	$ 1,616,321	$ 1,094,663
Percentage of Portfolio	11.00%	7.00%
Spain [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	1	1
Net book value	$ 1,057,616	$ 1,124,174
Total Loan Exposure	$ 1,063,416	$ 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	$ 150,302	$ 316,268
Total Loan Exposure	$ 149,942	$ 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	$ 343,650	$ 310,372
Total Loan Exposure	$ 345,741	$ 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	$ 69,982	$ 70,621
Total Loan Exposure	$ 70,249	$ 71,007
Germany [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	1	1
Net book value	$ 196,985	$ 11,585
Total Loan Exposure	$ 250,975	$ 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	21
Net book value	$ 3,089,082	2,594,835
Total Loan Exposure	$ 3,496,644	$ 2,994,300
Percentage of Portfolio	23.00%	19.00%