Loans Receivable, Net - Property Type and Geographic Distribution of Properties Securing Loans in Portfolio (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) SecurityLoan	Dec. 31, 2016 USD ($) SecurityLoan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	110	107
Net book value	$ 10,056,732	$ 8,692,978
Total Loan Exposure	$ 11,093,608	$ 9,756,734
Percentage of Portfolio	100.00%	100.00%
Office [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	53	55
Net book value	$ 5,773,972	$ 4,800,609
Total Loan Exposure	$ 5,807,170	$ 4,889,456
Percentage of Portfolio	53.00%	50.00%
Hotel [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	15	18
Net book value	$ 1,830,568	$ 1,889,732
Total Loan Exposure	$ 1,905,497	$ 1,957,334
Percentage of Portfolio	17.00%	20.00%
Multifamily [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	25	17
Net book value	$ 1,220,423	$ 817,387
Total Loan Exposure	$ 1,228,959	$ 819,781
Percentage of Portfolio	11.00%	8.00%
Retail [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	6	9
Net book value	$ 487,473	$ 769,813
Total Loan Exposure	$ 940,980	$ 1,173,592
Percentage of Portfolio	8.00%	12.00%
Condominium [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	2	2
Net book value	$ 142,342	$ 66,070
Total Loan Exposure	$ 268,751	$ 258,360
Percentage of Portfolio	2.00%	3.00%
Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	9	6
Net book value	$ 601,954	$ 349,367
Total Loan Exposure	$ 942,251	$ 658,211
Percentage of Portfolio	9.00%	7.00%
United States Northeast [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	26	26
Net book value	$ 2,857,948	$ 2,548,257
Total Loan Exposure	$ 2,871,219	$ 2,562,149
Percentage of Portfolio	26.00%	26.00%
United States West [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	29	22
Net book value	$ 2,672,069	$ 1,628,811
Total Loan Exposure	$ 2,816,276	$ 1,828,667
Percentage of Portfolio	24.00%	19.00%
United States Southeast [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	17	21
Net book value	$ 2,007,202	$ 1,492,530
Total Loan Exposure	$ 2,470,992	$ 1,899,748
Percentage of Portfolio	22.00%	19.00%
United States Midwest [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	9	7
Net book value	$ 856,559	$ 695,713
Total Loan Exposure	$ 862,578	$ 698,093
Percentage of Portfolio	8.00%	7.00%
United States Southwest [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	10	8
Net book value	$ 380,204	$ 380,639
Total Loan Exposure	$ 380,120	$ 379,766
Percentage of Portfolio	3.00%	4.00%
United States Northwest [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	2	3
Net book value	$ 283,381	$ 227,747
Total Loan Exposure	$ 286,221	$ 293,564
Percentage of Portfolio	3.00%	3.00%
United States [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	93	87
Net book value	$ 9,057,363	$ 6,973,697
Total Loan Exposure	$ 9,687,406	$ 7,661,987
Percentage of Portfolio	86.00%	78.00%
United Kingdom [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	6	9
Net book value	$ 440,317	$ 977,136
Total Loan Exposure	$ 794,789	$ 1,305,816
Percentage of Portfolio	7.00%	13.00%
Canada [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	7	8
Net book value	$ 415,893	$ 487,835
Total Loan Exposure	$ 412,343	$ 483,923
Percentage of Portfolio	4.00%	5.00%
Belgium [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	1
Net book value	$ 73,779
Total Loan Exposure	$ 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	$ 12,237	$ 204,241
Total Loan Exposure	$ 67,399	$ 254,644
Percentage of Portfolio	1.00%	3.00%
Netherlands [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	2	2
Net book value	$ 57,143	$ 50,069
Total Loan Exposure	$ 57,240	$ 50,364
Percentage of Portfolio	1.00%	1.00%
International [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	17	20
Net book value	$ 999,369	$ 1,719,281
Total Loan Exposure	$ 1,406,202	$ 2,094,747
Percentage of Portfolio	14.00%	22.00%