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, 2018 USD ($) SecurityLoan	Dec. 31, 2017 USD ($) SecurityLoan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	112	110
Net book value	$ 12,176,516	$ 10,056,732
Total Loan Exposure	$ 12,726,831	$ 11,093,608
Percentage of Portfolio	100.00%	100.00%
Office [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	49	53
Net book value	$ 5,956,675	$ 5,773,972
Total Loan Exposure	$ 6,006,534	$ 5,807,170
Percentage of Portfolio	47.00%	53.00%
Hotel [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	15	15
Net book value	$ 1,992,889	$ 1,830,568
Total Loan Exposure	$ 2,069,818	$ 1,905,497
Percentage of Portfolio	16.00%	17.00%
Multifamily [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	32	6
Net book value	$ 1,894,001	$ 487,473
Total Loan Exposure	$ 1,903,572	$ 940,980
Percentage of Portfolio	15.00%	8.00%
Spanish Assets [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	1
Net book value	$ 1,152,487
Total Loan Exposure	$ 1,160,400
Percentage of Portfolio	9.00%
Retail [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	4	25
Net book value	$ 336,787	$ 1,220,423
Total Loan Exposure	$ 337,334	$ 1,228,959
Percentage of Portfolio	3.00%	11.00%
Condominium [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	2	2
Net book value	$ 84,429	$ 142,342
Total Loan Exposure	$ 158,462	$ 268,751
Percentage of Portfolio	1.00%	2.00%
Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	9	9
Net book value	$ 759,248	$ 601,954
Total Loan Exposure	$ 1,090,711	$ 942,251
Percentage of Portfolio	9.00%	9.00%
United States Northeast [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	30	26
Net book value	$ 3,796,633	$ 2,857,948
Total Loan Exposure	$ 3,830,021	$ 2,871,219
Percentage of Portfolio	29.00%	26.00%
United States West [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	25	29
Net book value	$ 2,543,743	$ 2,672,069
Total Loan Exposure	$ 2,637,582	$ 2,816,276
Percentage of Portfolio	21.00%	24.00%
United States Southeast [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	18	17
Net book value	$ 2,025,381	$ 2,007,202
Total Loan Exposure	$ 2,034,852	$ 2,470,992
Percentage of Portfolio	16.00%	22.00%
United States Midwest [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	8	9
Net book value	$ 901,310	$ 856,559
Total Loan Exposure	$ 906,102	$ 862,578
Percentage of Portfolio	7.00%	8.00%
United States Southwest [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	13	10
Net book value	$ 477,809	$ 380,204
Total Loan Exposure	$ 480,792	$ 380,120
Percentage of Portfolio	4.00%	3.00%
United States Northwest [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	2	2
Net book value	$ 141,199	$ 283,381
Total Loan Exposure	$ 141,630	$ 286,221
Percentage of Portfolio	1.00%	3.00%
United States [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	96	93
Net book value	$ 9,886,075	$ 9,057,363
Total Loan Exposure	$ 10,030,979	$ 9,687,406
Percentage of Portfolio	78.00%	86.00%
Spain [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	1
Net book value	$ 1,152,487
Total Loan Exposure	$ 1,160,400
Percentage of Portfolio	9.00%
United Kingdom [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	5	6
Net book value	$ 492,790	$ 440,317
Total Loan Exposure	$ 837,688	$ 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	$ 335,957	$ 415,893
Total Loan Exposure	$ 332,841	$ 412,343
Percentage of Portfolio	3.00%	4.00%
Australia [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	2
Net book value	$ 218,286
Total Loan Exposure	$ 220,335
Percentage of Portfolio	2.00%
Belgium [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	1	1
Net book value	$ 71,494	$ 73,779
Total Loan Exposure	$ 71,945	$ 74,431
Percentage of Portfolio	1.00%	1.00%
Germany [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	1	1
Net book value	$ 11,814	$ 12,237
Total Loan Exposure	$ 65,000	$ 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	$ 7,613	$ 57,143
Total Loan Exposure	$ 7,643	$ 57,240
Percentage of Portfolio		1.00%
International [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	16	17
Net book value	$ 2,290,441	$ 999,369
Total Loan Exposure	$ 2,695,852	$ 1,406,202
Percentage of Portfolio	22.00%	14.00%