Loans Receivable, Net - Property Type and Geographic Distribution of Properties Securing Loans in Portfolio (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($) SecurityLoan	Dec. 31, 2017 USD ($) SecurityLoan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	125	110
Net book value	$ 14,191,200	$ 10,056,732
Total Loan Exposure	$ 14,740,844	$ 11,093,608
Percentage of Portfolio	100.00%	100.00%
Office [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	55	53
Net book value	$ 7,066,611	$ 5,773,972
Total Loan Exposure	$ 7,125,991	$ 5,807,170
Percentage of Portfolio	49.00%	53.00%
Hotel [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	18	15
Net book value	$ 2,541,283	$ 1,830,568
Total Loan Exposure	$ 2,623,161	$ 1,905,497
Percentage of Portfolio	18.00%	17.00%
Multifamily [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	34	25
Net book value	$ 1,961,522	$ 1,220,423
Total Loan Exposure	$ 1,973,084	$ 1,228,959
Percentage of Portfolio	13.00%	11.00%
Spanish Assets [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	1
Net book value	$ 1,124,174
Total Loan Exposure	$ 1,131,334
Percentage of Portfolio	8.00%
Retail [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	4	6
Net book value	$ 348,961	$ 487,473
Total Loan Exposure	$ 350,112	$ 940,980
Percentage of Portfolio	2.00%	8.00%
Condominium [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	3	2
Net book value	$ 92,557	$ 142,342
Total Loan Exposure	$ 154,766	$ 268,751
Percentage of Portfolio	1.00%	2.00%
Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	3	9
Net book value	$ 169,513	$ 601,954
Total Loan Exposure	$ 489,742	$ 942,251
Percentage of Portfolio	3.00%	9.00%
Industrial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	5
Net book value	$ 609,261
Total Loan Exposure	$ 613,774
Percentage of Portfolio	4.00%
Self-Storage [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	2
Net book value	$ 277,318
Total Loan Exposure	$ 278,880
Percentage of Portfolio	2.00%
United States Northeast [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	32	26
Net book value	$ 4,322,114	$ 2,857,948
Total Loan Exposure	$ 4,359,938	$ 2,871,219
Percentage of Portfolio	31.00%	26.00%
United States West [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	29	29
Net book value	$ 3,137,072	$ 2,672,069
Total Loan Exposure	$ 3,222,706	$ 2,816,276
Percentage of Portfolio	22.00%	24.00%
United States Southeast [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	19	17
Net book value	$ 2,258,033	$ 2,007,202
Total Loan Exposure	$ 2,271,664	$ 2,470,992
Percentage of Portfolio	15.00%	22.00%
United States Midwest [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	9	9
Net book value	$ 1,161,637	$ 856,559
Total Loan Exposure	$ 1,170,619	$ 862,578
Percentage of Portfolio	8.00%	8.00%
United States Southwest [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	13	10
Net book value	$ 478,665	$ 380,204
Total Loan Exposure	$ 481,745	$ 380,120
Percentage of Portfolio	3.00%	3.00%
United States Northwest [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	4	2
Net book value	$ 238,844	$ 283,381
Total Loan Exposure	$ 239,872	$ 286,221
Percentage of Portfolio	2.00%	3.00%
United States [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	106	93
Net book value	$ 11,596,365	$ 9,057,363
Total Loan Exposure	$ 11,746,544	$ 9,687,406
Percentage of Portfolio	81.00%	86.00%
Spain [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	1
Net book value	$ 1,124,174
Total Loan Exposure	$ 1,131,334
Percentage of Portfolio	8.00%
United Kingdom [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	7	6
Net book value	$ 754,299	$ 440,317
Total Loan Exposure	$ 1,094,663	$ 794,789
Percentage of Portfolio	7.00%	7.00%
Canada [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	5	7
Net book value	$ 316,268	$ 415,893
Total Loan Exposure	$ 313,229	$ 412,343
Percentage of Portfolio	2.00%	4.00%
Australia [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	3
Net book value	$ 310,372
Total Loan Exposure	$ 312,893
Percentage of Portfolio	2.00%
Belgium [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	1	1
Net book value	$ 70,621	$ 73,779
Total Loan Exposure	$ 71,007	$ 74,431
Percentage of Portfolio		1.00%
Germany [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	1	1
Net book value	$ 11,585	$ 12,237
Total Loan Exposure	$ 63,637	$ 67,399
Percentage of Portfolio		1.00%
Netherlands [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	1	2
Net book value	$ 7,516	$ 57,143
Total Loan Exposure	$ 7,537	$ 57,240
Percentage of Portfolio		1.00%
International [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	19	17
Net book value	$ 2,594,835	$ 999,369
Total Loan Exposure	$ 2,994,300	$ 1,406,202
Percentage of Portfolio	19.00%	14.00%